Add the __libc_single_threaded variable

The variable is placed in libc.so, and it can be true only in
an outer libc, not libcs loaded via dlmopen or static dlopen.
Since thread creation from inner namespaces does not work,
pthread_create can update __libc_single_threaded directly.

Using __libc_early_init and its initial flag, implementation of this
variable is very straightforward.  A future version may reset the flag
during fork (but not in an inner namespace), or after joining all
threads except one.

Reviewed-by: DJ Delorie <dj@redhat.com>

51 files changed, 663 insertions, 8 deletions

diff --git a/NEWS b/NEWS
index a12b9378e7..839934d649 100644
--- a/NEWS
+++ b/NEWS
@@ -41,6 +41,12 @@ Major new features:
   The GNU C Library manual has details on integration of Restartable
   Sequences.
 
+* The GNU C Library now provides the header file <sys/single_threaded.h>
+  which declares the variable __libc_single_threaded.  Applications are
+  encouraged to use this variable for single-thread optimizations,
+  instead of weak references to symbols historically defined in
+  libpthread.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The deprecated <sys/sysctl.h> header and the sysctl function have been
diff --git a/elf/Makefile b/elf/Makefile
index 6fe1df90bb..81a696c3ef 100644
--- a/elf/Makefile
+++ b/elf/Makefile
@@ -155,7 +155,9 @@ endif
 tests-static-normal := tst-leaks1-static tst-array1-static tst-array5-static \
 	       tst-dl-iter-static \
 	       tst-tlsalign-static tst-tlsalign-extern-static \
-	       tst-linkall-static tst-env-setuid tst-env-setuid-tunables
+	       tst-linkall-static tst-env-setuid tst-env-setuid-tunables \
+	       tst-single_threaded-static tst-single_threaded-pthread-static
+
 tests-static-internal := tst-tls1-static tst-tls2-static \
 	       tst-ptrguard1-static tst-stackguard1-static \
 	       tst-tls1-static-non-pie tst-libc_dlvsym-static
@@ -174,9 +176,11 @@ tests-internal := tst-tls1 tst-tls2 $(tests-static-internal)
 tests-static := $(tests-static-normal) $(tests-static-internal)
 
 ifeq (yes,$(build-shared))
-tests-static += tst-tls9-static
-tst-tls9-static-ENV = \
-       LD_LIBRARY_PATH=$(objpfx):$(common-objpfx):$(common-objpfx)dlfcn
+tests-static += tst-tls9-static tst-single_threaded-static-dlopen
+static-dlopen-environment = \
+  LD_LIBRARY_PATH=$(objpfx):$(common-objpfx):$(common-objpfx)dlfcn
+tst-tls9-static-ENV = $(static-dlopen-environment)
+tst-single_threaded-static-dlopen-ENV = $(static-dlopen-environment)
 
 tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 constload1 order noload filter \
@@ -204,7 +208,8 @@ tests += restest1 preloadtest loadfail multiload origtest resolvfail \
 	 tst-dlopen-self tst-auditmany tst-initfinilazyfail tst-dlopenfail \
 	 tst-dlopenfail-2 \
 	 tst-filterobj tst-filterobj-dlopen tst-auxobj tst-auxobj-dlopen \
-	 tst-audit14 tst-audit15 tst-audit16
+	 tst-audit14 tst-audit15 tst-audit16 \
+	 tst-single_threaded tst-single_threaded-pthread
 #	 reldep9
 tests-internal += loadtest unload unload2 circleload1 \
 	 neededtest neededtest2 neededtest3 neededtest4 \
@@ -317,7 +322,9 @@ modules-names = testobj1 testobj2 testobj3 testobj4 testobj5 testobj6 \
 		tst-dlopenfailmod1 tst-dlopenfaillinkmod tst-dlopenfailmod2 \
 		tst-dlopenfailmod3 tst-ldconfig-ld-mod \
 		tst-filterobj-flt tst-filterobj-aux tst-filterobj-filtee \
-		tst-auditlogmod-1 tst-auditlogmod-2 tst-auditlogmod-3
+		tst-auditlogmod-1 tst-auditlogmod-2 tst-auditlogmod-3 \
+		tst-single_threaded-mod1 tst-single_threaded-mod2 \
+		tst-single_threaded-mod3 tst-single_threaded-mod4
 # Most modules build with _ISOMAC defined, but those filtered out
 # depend on internal headers.
 modules-names-tests = $(filter-out ifuncmod% tst-libc_dlvsym-dso tst-tlsmod%,\
@@ -1748,3 +1755,17 @@ $(objpfx)tst-auxobj: $(objpfx)tst-filterobj-aux.so
 $(objpfx)tst-auxobj-dlopen: $(libdl)
 $(objpfx)tst-auxobj.out: $(objpfx)tst-filterobj-filtee.so
 $(objpfx)tst-auxobj-dlopen.out: $(objpfx)tst-filterobj-filtee.so
+
+$(objpfx)tst-single_threaded: $(objpfx)tst-single_threaded-mod1.so $(libdl)
+$(objpfx)tst-single_threaded.out: \
+  $(objpfx)tst-single_threaded-mod2.so $(objpfx)tst-single_threaded-mod3.so
+$(objpfx)tst-single_threaded-static-dlopen: \
+  $(objpfx)tst-single_threaded-mod1.o $(common-objpfx)dlfcn/libdl.a
+$(objpfx)tst-single_threaded-static-dlopen.out: \
+  $(objpfx)tst-single_threaded-mod2.so
+$(objpfx)tst-single_threaded-pthread: \
+  $(objpfx)tst-single_threaded-mod1.so $(libdl) $(shared-thread-library)
+$(objpfx)tst-single_threaded-pthread.out: \
+  $(objpfx)tst-single_threaded-mod2.so $(objpfx)tst-single_threaded-mod3.so \
+  $(objpfx)tst-single_threaded-mod4.so
+$(objpfx)tst-single_threaded-pthread-static: $(static-thread-library)
diff --git a/elf/libc_early_init.c b/elf/libc_early_init.c
index f0fcf6448e..86da66d5e0 100644
--- a/elf/libc_early_init.c
+++ b/elf/libc_early_init.c
@@ -19,13 +19,18 @@
 #include <ctype.h>
 #include <libc-early-init.h>
 #include <rseq-internal.h>
+#include <sys/single_threaded.h>
 
 void
 __libc_early_init (_Bool initial)
 {
   /* Initialize ctype data.  */
   __ctype_init ();
+
   /* Register rseq ABI to the kernel for the main program's libc.   */
   if (initial)
     rseq_register_current_thread ();
+
+  /* Only the outer namespace is marked as single-threaded.  */
+  __libc_single_threaded = initial;
 }
diff --git a/elf/tst-single_threaded-mod1.c b/elf/tst-single_threaded-mod1.c
new file mode 100644
index 0000000000..a66de65679
--- /dev/null
+++ b/elf/tst-single_threaded-mod1.c
@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 1.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_1 (void)
+{
+  return __libc_single_threaded;
+}
diff --git a/elf/tst-single_threaded-mod2.c b/elf/tst-single_threaded-mod2.c
new file mode 100644
index 0000000000..74d1e2a6f4
--- /dev/null
+++ b/elf/tst-single_threaded-mod2.c
@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 2.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_2 (void)
+{
+  return __libc_single_threaded;
+}
diff --git a/elf/tst-single_threaded-mod3.c b/elf/tst-single_threaded-mod3.c
new file mode 100644
index 0000000000..ac5e49cfa5
--- /dev/null
+++ b/elf/tst-single_threaded-mod3.c
@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 3.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_3 (void)
+{
+  return __libc_single_threaded;
+}
diff --git a/elf/tst-single_threaded-mod4.c b/elf/tst-single_threaded-mod4.c
new file mode 100644
index 0000000000..9f3a8ecef1
--- /dev/null
+++ b/elf/tst-single_threaded-mod4.c
@@ -0,0 +1,25 @@
+/* Test support for single-thread optimizations.  Shared object 4.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+_Bool
+single_threaded_4 (void)
+{
+  return __libc_single_threaded;
+}
diff --git a/elf/tst-single_threaded-pthread-static.c b/elf/tst-single_threaded-pthread-static.c
new file mode 100644
index 0000000000..9f383a0191
--- /dev/null
+++ b/elf/tst-single_threaded-pthread-static.c
@@ -0,0 +1,86 @@
+/* Test support for single-thread optimizations.  With threads, static version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This test is a stripped-down version of
+   tst-single_threaded-pthread.c, without any loading of dynamic
+   objects.  */
+
+#include <stdio.h>
+#include <support/check.h>
+#include <support/xthread.h>
+#include <sys/single_threaded.h>
+
+/* First barrier synchronizes main thread, thread 1, thread 2.  */
+static pthread_barrier_t barrier1;
+
+/* Second barrier synchronizes main thread, thread 2.  */
+static pthread_barrier_t barrier2;
+
+static void *
+threadfunc (void *closure)
+{
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* Wait for the main thread and the other thread.  */
+  xpthread_barrier_wait (&barrier1);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* Second thread waits on second barrier, too.  */
+  if (closure != NULL)
+    xpthread_barrier_wait (&barrier2);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  return NULL;
+}
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+
+  /* Two threads plus main thread.  */
+  xpthread_barrier_init (&barrier1, NULL, 3);
+
+  /* Main thread and second thread.  */
+  xpthread_barrier_init (&barrier2, NULL, 2);
+
+  pthread_t thr1 = xpthread_create (NULL, threadfunc, NULL);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  pthread_t thr2 = xpthread_create (NULL, threadfunc, &thr2);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  xpthread_barrier_wait (&barrier1);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* Join first thread.  This should not bring us back into
+     single-threaded mode.  */
+  xpthread_join (thr1);
+  TEST_VERIFY (!__libc_single_threaded);
+
+  /* We may be back in single-threaded mode after joining both
+     threads, but this is not guaranteed.  */
+  xpthread_barrier_wait (&barrier2);
+  xpthread_join (thr2);
+  printf ("info: __libc_single_threaded after joining all threads: %d\n",
+          __libc_single_threaded);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-single_threaded-pthread.c b/elf/tst-single_threaded-pthread.c
new file mode 100644
index 0000000000..b7112b03c1
--- /dev/null
+++ b/elf/tst-single_threaded-pthread.c
@@ -0,0 +1,174 @@
+/* Test support for single-thread optimizations.  With threads.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/xdlfcn.h>
+#include <support/xthread.h>
+#include <sys/single_threaded.h>
+
+/* First barrier synchronizes main thread, thread 1, thread 2.  */
+static pthread_barrier_t barrier1;
+
+/* Second barrier synchronizes main thread, thread 2.  */
+static pthread_barrier_t barrier2;
+
+/* Defined in tst-single-threaded-mod1.so.  */
+_Bool single_threaded_1 (void);
+
+/* Initialized via dlsym.  */
+static _Bool (*single_threaded_2) (void);
+static _Bool (*single_threaded_3) (void);
+static _Bool (*single_threaded_4) (void);
+
+static void *
+threadfunc (void *closure)
+{
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+
+  /* Wait until the main thread loads more functions.  */
+  xpthread_barrier_wait (&barrier1);
+
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+
+  /* Second thread waits on second barrier, too.  */
+  if (closure != NULL)
+    xpthread_barrier_wait (&barrier2);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+
+  return NULL;
+}
+
+/* Used for closure arguments to the subprocess function.  */
+static char expected_false = 0;
+static char expected_true = 1;
+
+/* A subprocess inherits currently inherits the single-threaded state
+   of the parent process.  */
+static void
+subprocess (void *closure)
+{
+  const char *expected = closure;
+  TEST_COMPARE (__libc_single_threaded, *expected);
+  TEST_COMPARE (single_threaded_1 (), *expected);
+  if (single_threaded_2 != NULL)
+    TEST_COMPARE (single_threaded_2 (), *expected);
+  if (single_threaded_3 != NULL)
+    TEST_COMPARE (single_threaded_3 (), *expected);
+  if (single_threaded_4 != NULL)
+    TEST_VERIFY (!single_threaded_4 ());
+}
+
+static int
+do_test (void)
+{
+  printf ("info: main __libc_single_threaded address: %p\n",
+          &__libc_single_threaded);
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  support_isolate_in_subprocess (subprocess, &expected_true);
+
+  void *handle_mod2 = xdlopen ("tst-single_threaded-mod2.so", RTLD_LAZY);
+  single_threaded_2 = xdlsym (handle_mod2, "single_threaded_2");
+  TEST_VERIFY (single_threaded_2 ());
+
+  /* Two threads plus main thread.  */
+  xpthread_barrier_init (&barrier1, NULL, 3);
+
+  /* Main thread and second thread.  */
+  xpthread_barrier_init (&barrier2, NULL, 2);
+
+  pthread_t thr1 = xpthread_create (NULL, threadfunc, NULL);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  pthread_t thr2 = xpthread_create (NULL, threadfunc, &thr2);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Delayed library load, while already multi-threaded.  */
+  void *handle_mod3 = xdlopen ("tst-single_threaded-mod3.so", RTLD_LAZY);
+  single_threaded_3 = xdlsym (handle_mod3, "single_threaded_3");
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Same with dlmopen.  */
+  void *handle_mod4 = dlmopen (LM_ID_NEWLM, "tst-single_threaded-mod4.so",
+                               RTLD_LAZY);
+  single_threaded_4 = xdlsym (handle_mod4, "single_threaded_4");
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Run the newly loaded functions from the other threads as
+     well.  */
+  xpthread_barrier_wait (&barrier1);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* Join first thread.  This should not bring us back into
+     single-threaded mode.  */
+  xpthread_join (thr1);
+  TEST_VERIFY (!__libc_single_threaded);
+  TEST_VERIFY (!single_threaded_1 ());
+  TEST_VERIFY (!single_threaded_2 ());
+  TEST_VERIFY (!single_threaded_3 ());
+  TEST_VERIFY (!single_threaded_4 ());
+  support_isolate_in_subprocess (subprocess, &expected_false);
+
+  /* We may be back in single-threaded mode after joining both
+     threads, but this is not guaranteed.  */
+  xpthread_barrier_wait (&barrier2);
+  xpthread_join (thr2);
+  printf ("info: __libc_single_threaded after joining all threads: %d\n",
+          __libc_single_threaded);
+
+  xdlclose (handle_mod4);
+  xdlclose (handle_mod3);
+  xdlclose (handle_mod2);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-single_threaded-static-dlopen.c b/elf/tst-single_threaded-static-dlopen.c
new file mode 100644
index 0000000000..f33588312d
--- /dev/null
+++ b/elf/tst-single_threaded-static-dlopen.c
@@ -0,0 +1,57 @@
+/* Test support for single-thread optimizations.  No threads, static dlopen.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* In a static dlopen scenario, the single-threaded optimization is
+   not possible because their is no globally shared dynamic linker
+   across all namespaces.  */
+
+#include <stddef.h>
+#include <support/check.h>
+#include <support/xdlfcn.h>
+#include <sys/single_threaded.h>
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+
+  /* Defined in tst-single-threaded-mod1.o.  */
+  extern _Bool single_threaded_1 (void);
+  TEST_VERIFY (single_threaded_1 ());
+
+  /* A failed dlopen does not change the multi-threaded status.  */
+  TEST_VERIFY (dlopen ("tst-single_threaded-does-not-exist.so", RTLD_LAZY)
+               == NULL);
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+
+  /* And neither does a successful dlopen for outer (static) libc.  */
+  void *handle_mod2 = xdlopen ("tst-single_threaded-mod2.so", RTLD_LAZY);
+  _Bool (*single_threaded_2) (void)
+    = xdlsym (handle_mod2, "single_threaded_2");
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  /* The inner libc always assumes multi-threaded use.  */
+  TEST_VERIFY (!single_threaded_2 ());
+
+  xdlclose (handle_mod2);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-single_threaded-static.c b/elf/tst-single_threaded-static.c
new file mode 100644
index 0000000000..593da8866d
--- /dev/null
+++ b/elf/tst-single_threaded-static.c
@@ -0,0 +1,29 @@
+/* Test support for single-thread optimizations.  Static, no threads.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/check.h>
+#include <sys/single_threaded.h>
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/elf/tst-single_threaded.c b/elf/tst-single_threaded.c
new file mode 100644
index 0000000000..2c36342779
--- /dev/null
+++ b/elf/tst-single_threaded.c
@@ -0,0 +1,70 @@
+/* Test support for single-thread optimizations.  No threads.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stddef.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/xdlfcn.h>
+#include <sys/single_threaded.h>
+
+/* Defined in tst-single-threaded-mod1.so.  */
+extern _Bool single_threaded_1 (void);
+
+/* Initialized via dlsym.  */
+_Bool (*single_threaded_2) (void);
+_Bool (*single_threaded_3) (void);
+
+static void
+subprocess (void *closure)
+{
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  if (single_threaded_2 != NULL)
+    TEST_VERIFY (single_threaded_2 ());
+  if (single_threaded_3 != NULL)
+    TEST_VERIFY (!single_threaded_3 ());
+}
+
+static int
+do_test (void)
+{
+  TEST_VERIFY (__libc_single_threaded);
+  TEST_VERIFY (single_threaded_1 ());
+  support_isolate_in_subprocess (subprocess, NULL);
+
+  void *handle_mod2 = xdlopen ("tst-single_threaded-mod2.so", RTLD_LAZY);
+  single_threaded_2 = xdlsym (handle_mod2, "single_threaded_2");
+  TEST_VERIFY (single_threaded_2 ());
+  support_isolate_in_subprocess (subprocess, NULL);
+
+  /* The current implementation treats the inner namespace as
+     multi-threaded.  */
+  void *handle_mod3 = dlmopen (LM_ID_NEWLM, "tst-single_threaded-mod3.so",
+                               RTLD_LAZY);
+  single_threaded_3 = xdlsym (handle_mod3, "single_threaded_3");
+  TEST_VERIFY (!single_threaded_3 ());
+  support_isolate_in_subprocess (subprocess, NULL);
+
+  xdlclose (handle_mod3);
+  xdlclose (handle_mod2);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/htl/pt-create.c b/htl/pt-create.c
index f501a12017..7ac875cbf7 100644
--- a/htl/pt-create.c
+++ b/htl/pt-create.c
@@ -24,6 +24,7 @@
 
 #include <atomic.h>
 #include <hurd/resource.h>
+#include <sys/single_threaded.h>
 
 #include <pt-internal.h>
 #include <pthreadP.h>
@@ -104,6 +105,10 @@ __pthread_create_internal (struct __pthread **thread,
   sigset_t sigset;
   size_t stacksize;
 
+  /* Avoid a data race in the multi-threaded case.  */
+  if (__libc_single_threaded)
+    __libc_single_threaded = 0;
+
   /* Allocate a new thread structure.  */
   err = __pthread_alloc (&pthread);
   if (err)
diff --git a/include/sys/single_threaded.h b/include/sys/single_threaded.h
new file mode 100644
index 0000000000..18f6972482
--- /dev/null
+++ b/include/sys/single_threaded.h
@@ -0,0 +1 @@
+#include <misc/sys/single_threaded.h>
diff --git a/misc/Makefile b/misc/Makefile
index 67c5237f97..58959f6913 100644
--- a/misc/Makefile
+++ b/misc/Makefile
@@ -37,7 +37,8 @@ headers	:= sys/uio.h bits/uio-ext.h bits/uio_lim.h \
 	   bits/syslog.h bits/syslog-ldbl.h bits/syslog-path.h bits/error.h \
 	   bits/select2.h bits/hwcap.h sys/auxv.h \
 	   sys/sysmacros.h bits/sysmacros.h bits/types/struct_iovec.h \
-	   bits/err-ldbl.h bits/error-ldbl.h
+	   bits/err-ldbl.h bits/error-ldbl.h \
+	   sys/single_threaded.h
 
 routines := brk sbrk sstk ioctl \
 	    readv writev preadv preadv64 pwritev pwritev64 \
@@ -72,7 +73,7 @@ routines := brk sbrk sstk ioctl \
 	    fgetxattr flistxattr fremovexattr fsetxattr getxattr \
 	    listxattr lgetxattr llistxattr lremovexattr lsetxattr \
 	    removexattr setxattr getauxval ifunc-impl-list makedev \
-	    allocate_once fd_to_filename
+	    allocate_once fd_to_filename single_threaded
 
 generated += tst-error1.mtrace tst-error1-mem.out \
   tst-allocate_once.mtrace tst-allocate_once-mem.out
diff --git a/misc/Versions b/misc/Versions
index e749582369..95666f6548 100644
--- a/misc/Versions
+++ b/misc/Versions
@@ -161,6 +161,9 @@ libc {
   GLIBC_2.30 {
     twalk_r;
   }
+  GLIBC_2.32 {
+    __libc_single_threaded;
+  }
   GLIBC_PRIVATE {
     __madvise;
     __mktemp;
diff --git a/misc/single_threaded.c b/misc/single_threaded.c
new file mode 100644
index 0000000000..6d975c0eaa
--- /dev/null
+++ b/misc/single_threaded.c
@@ -0,0 +1,27 @@
+/* Support for single-thread optimizations.  Statically linked version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sys/single_threaded.h>
+
+/* In dynamically linked programs, this variable is initialized in
+   __libc_early_init (as false for inner libcs).  */
+#ifdef SHARED
+char __libc_single_threaded;
+#else
+char __libc_single_threaded = 1;
+#endif
diff --git a/misc/sys/single_threaded.h b/misc/sys/single_threaded.h
new file mode 100644
index 0000000000..648fc2e00a
--- /dev/null
+++ b/misc/sys/single_threaded.h
@@ -0,0 +1,33 @@
+/* Support for single-thread optimizations.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_SINGLE_THREADED_H
+#define _SYS_SINGLE_THREADED_H
+
+#include <features.h>
+
+__BEGIN_DECLS
+
+/* If this variable is non-zero, then the current thread is the only
+   thread in the process image.  If it is zero, the process might be
+   multi-threaded.  */
+extern char __libc_single_threaded;
+
+__END_DECLS
+
+#endif /* _SYS_SINGLE_THREADED_H */
diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index f348a6f6dd..e05013e317 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -35,6 +35,7 @@
 #include <tls-setup.h>
 #include <rseq-internal.h>
 #include "libioP.h"
+#include <sys/single_threaded.h>
 
 #include <shlib-compat.h>
 
@@ -625,6 +626,10 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
 {
   STACK_VARIABLES;
 
+  /* Avoid a data race in the multi-threaded case.  */
+  if (__libc_single_threaded)
+    __libc_single_threaded = 0;
+
   const struct pthread_attr *iattr = (struct pthread_attr *) attr;
   union pthread_attr_transparent default_attr;
   bool destroy_default_attr = false;
diff --git a/sysdeps/generic/libc.abilist b/sysdeps/generic/libc.abilist
index e69de29bb2..8ca9b93c2f 100644
--- a/sysdeps/generic/libc.abilist
+++ b/sysdeps/generic/libc.abilist
@@ -0,0 +1 @@
+GLIBC_2.32 __libc_single_threaded D 0x1
diff --git a/sysdeps/mach/hurd/i386/libc.abilist b/sysdeps/mach/hurd/i386/libc.abilist
index 6400885d1d..ea985b0e41 100644
--- a/sysdeps/mach/hurd/i386/libc.abilist
+++ b/sysdeps/mach/hurd/i386/libc.abilist
@@ -2181,6 +2181,7 @@ GLIBC_2.3.4 setsourcefilter F
 GLIBC_2.3.4 xdr_quad_t F
 GLIBC_2.3.4 xdr_u_quad_t F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 mach_print F
 GLIBC_2.32 mremap F
 GLIBC_2.32 thrd_current F
diff --git a/sysdeps/unix/sysv/linux/aarch64/libc.abilist b/sysdeps/unix/sysv/linux/aarch64/libc.abilist
index e6e4f087eb..bb0af758de 100644
--- a/sysdeps/unix/sysv/linux/aarch64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/aarch64/libc.abilist
@@ -2149,6 +2149,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/alpha/libc.abilist b/sysdeps/unix/sysv/linux/alpha/libc.abilist
index 835897876b..d4d1cd4845 100644
--- a/sysdeps/unix/sysv/linux/alpha/libc.abilist
+++ b/sysdeps/unix/sysv/linux/alpha/libc.abilist
@@ -2231,6 +2231,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/arm/be/libc.abilist b/sysdeps/unix/sysv/linux/arm/be/libc.abilist
index f7a61fc73a..d71beafec9 100644
--- a/sysdeps/unix/sysv/linux/arm/be/libc.abilist
+++ b/sysdeps/unix/sysv/linux/arm/be/libc.abilist
@@ -133,6 +133,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/arm/le/libc.abilist b/sysdeps/unix/sysv/linux/arm/le/libc.abilist
index c84ab6e5c3..e68a49288c 100644
--- a/sysdeps/unix/sysv/linux/arm/le/libc.abilist
+++ b/sysdeps/unix/sysv/linux/arm/le/libc.abilist
@@ -130,6 +130,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/csky/libc.abilist b/sysdeps/unix/sysv/linux/csky/libc.abilist
index 2fadebd7ee..ed9222b4c4 100644
--- a/sysdeps/unix/sysv/linux/csky/libc.abilist
+++ b/sysdeps/unix/sysv/linux/csky/libc.abilist
@@ -2093,6 +2093,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/hppa/libc.abilist b/sysdeps/unix/sysv/linux/hppa/libc.abilist
index c28604ffc7..8d1073a60a 100644
--- a/sysdeps/unix/sysv/linux/hppa/libc.abilist
+++ b/sysdeps/unix/sysv/linux/hppa/libc.abilist
@@ -2052,6 +2052,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/i386/libc.abilist b/sysdeps/unix/sysv/linux/i386/libc.abilist
index 4042d2b4e1..8fb09270c3 100644
--- a/sysdeps/unix/sysv/linux/i386/libc.abilist
+++ b/sysdeps/unix/sysv/linux/i386/libc.abilist
@@ -2218,6 +2218,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/ia64/libc.abilist b/sysdeps/unix/sysv/linux/ia64/libc.abilist
index 13a38cbafd..a3e6e48cd4 100644
--- a/sysdeps/unix/sysv/linux/ia64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/ia64/libc.abilist
@@ -2084,6 +2084,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist b/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
index 231e41ed37..cbc6fce155 100644
--- a/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
+++ b/sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist
@@ -134,6 +134,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist b/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
index b244e2a327..7be3214348 100644
--- a/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
+++ b/sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist
@@ -2164,6 +2164,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/microblaze/be/libc.abilist b/sysdeps/unix/sysv/linux/microblaze/be/libc.abilist
index 8c53e8b512..51468ddf55 100644
--- a/sysdeps/unix/sysv/linux/microblaze/be/libc.abilist
+++ b/sysdeps/unix/sysv/linux/microblaze/be/libc.abilist
@@ -2144,6 +2144,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/microblaze/le/libc.abilist b/sysdeps/unix/sysv/linux/microblaze/le/libc.abilist
index fa7df5a6d8..61eb9920ec 100644
--- a/sysdeps/unix/sysv/linux/microblaze/le/libc.abilist
+++ b/sysdeps/unix/sysv/linux/microblaze/le/libc.abilist
@@ -2141,6 +2141,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
index 831251f1fd..e85b64a9db 100644
--- a/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist
@@ -2135,6 +2135,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
index 6f3c014ed1..72c54333ec 100644
--- a/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist
@@ -2133,6 +2133,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
index c3a3931e55..edd15902a5 100644
--- a/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist
@@ -2141,6 +2141,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
index bdc9b416b2..0f35a63433 100644
--- a/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist
@@ -2135,6 +2135,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/nios2/libc.abilist b/sysdeps/unix/sysv/linux/nios2/libc.abilist
index d8573f6bbe..5a4f50b208 100644
--- a/sysdeps/unix/sysv/linux/nios2/libc.abilist
+++ b/sysdeps/unix/sysv/linux/nios2/libc.abilist
@@ -2182,6 +2182,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
index 77309e8044..3de2243b63 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist
@@ -2191,6 +2191,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
index 84da7446ca..abf8fa58a1 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist
@@ -2224,6 +2224,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist
index 219316b5f4..c82911c9ef 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/libc.abilist
@@ -2054,6 +2054,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist
index 05c99997a3..f64c315a39 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist
@@ -2279,6 +2279,7 @@ GLIBC_2.32 __isoc99_vsscanfieee128 F
 GLIBC_2.32 __isoc99_vswscanfieee128 F
 GLIBC_2.32 __isoc99_vwscanfieee128 F
 GLIBC_2.32 __isoc99_wscanfieee128 F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __obstack_printf_chkieee128 F
 GLIBC_2.32 __obstack_printfieee128 F
 GLIBC_2.32 __obstack_vprintf_chkieee128 F
diff --git a/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist b/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
index 22db101803..991e361ae7 100644
--- a/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist
@@ -2111,6 +2111,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist b/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
index 256ce370ae..76d859aad7 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist
@@ -2189,6 +2189,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist b/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
index 5d6304393b..a4c5b38385 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist
@@ -2090,6 +2090,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/sh/be/libc.abilist b/sysdeps/unix/sysv/linux/sh/be/libc.abilist
index bff6c48ae4..3df8bbe813 100644
--- a/sysdeps/unix/sysv/linux/sh/be/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sh/be/libc.abilist
@@ -2059,6 +2059,7 @@ GLIBC_2.30 twalk_r F
 GLIBC_2.31 msgctl F
 GLIBC_2.31 semctl F
 GLIBC_2.31 shmctl F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/sh/le/libc.abilist b/sysdeps/unix/sysv/linux/sh/le/libc.abilist
index e55f46a5d6..ade2fc94e5 100644
--- a/sysdeps/unix/sysv/linux/sh/le/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sh/le/libc.abilist
@@ -2056,6 +2056,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist b/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
index 96e76c76a3..0dea9b46b0 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist
@@ -2180,6 +2180,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist b/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
index e6bfb396b2..32f86b96db 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist
@@ -2107,6 +2107,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
index 168ca03aa2..90f3c1d2f0 100644
--- a/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
+++ b/sysdeps/unix/sysv/linux/x86_64/64/libc.abilist
@@ -2065,6 +2065,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F
diff --git a/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist b/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
index 82a5089792..8dbe6d3ed2 100644
--- a/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
+++ b/sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist
@@ -2162,6 +2162,7 @@ GLIBC_2.30 getdents64 F
 GLIBC_2.30 gettid F
 GLIBC_2.30 tgkill F
 GLIBC_2.30 twalk_r F
+GLIBC_2.32 __libc_single_threaded D 0x1
 GLIBC_2.32 __rseq_abi T 0x20
 GLIBC_2.32 pthread_attr_getsigmask_np F
 GLIBC_2.32 pthread_attr_setaffinity_np F