12 files changed, 53 insertions, 0 deletions
diff --git a/libio/Versions b/libio/Versions
index 2a1d6e6c85..77123347e3 100644
--- a/libio/Versions
+++ b/libio/Versions
@@ -155,5 +155,8 @@ libc {
   GLIBC_PRIVATE {
     # Used by NPTL and librt
     __libc_fatal;
+
+    # Used by NPTL
+    _IO_enable_locks;
   }
 }
diff --git a/libio/feof.c b/libio/feof.c
index 9712a81e78..8890a5f51f 100644
--- a/libio/feof.c
+++ b/libio/feof.c
@@ -32,6 +32,8 @@ _IO_feof (_IO_FILE *fp)
 {
   int result;
   CHECK_FILE (fp, EOF);
+  if (!_IO_need_lock (fp))
+    return _IO_feof_unlocked (fp);
   _IO_flockfile (fp);
   result = _IO_feof_unlocked (fp);
   _IO_funlockfile (fp);
diff --git a/libio/ferror.c b/libio/ferror.c
index 01e3bd8e2b..d10fcd9fff 100644
--- a/libio/ferror.c
+++ b/libio/ferror.c
@@ -32,6 +32,8 @@ _IO_ferror (_IO_FILE *fp)
 {
   int result;
   CHECK_FILE (fp, EOF);
+  if (!_IO_need_lock (fp))
+    return _IO_ferror_unlocked (fp);
   _IO_flockfile (fp);
   result = _IO_ferror_unlocked (fp);
   _IO_funlockfile (fp);
diff --git a/libio/fputc.c b/libio/fputc.c
index a7cd682fe2..b72305c06f 100644
--- a/libio/fputc.c
+++ b/libio/fputc.c
@@ -32,6 +32,8 @@ fputc (int c, _IO_FILE *fp)
 {
   int result;
   CHECK_FILE (fp, EOF);
+  if (!_IO_need_lock (fp))
+    return _IO_putc_unlocked (c, fp);
   _IO_acquire_lock (fp);
   result = _IO_putc_unlocked (c, fp);
   _IO_release_lock (fp);
diff --git a/libio/genops.c b/libio/genops.c
index a466cfa337..6ad7346cae 100644
--- a/libio/genops.c
+++ b/libio/genops.c
@@ -570,11 +570,39 @@ _IO_init (_IO_FILE *fp, int flags)
   _IO_init_internal (fp, flags);
 }
 
+static int stdio_needs_locking;
+
+/* In a single-threaded process most stdio locks can be omitted.  After
+   _IO_enable_locks is called, locks are not optimized away any more.
+   It must be first called while the process is still single-threaded.
+
+   This lock optimization can be disabled on a per-file basis by setting
+   _IO_FLAGS2_NEED_LOCK, because a file can have user-defined callbacks
+   or can be locked with flockfile and then a thread may be created
+   between a lock and unlock, so omitting the lock is not valid.
+
+   Here we have to make sure that the flag is set on all existing files
+   and files created later.  */
+void
+_IO_enable_locks (void)
+{
+  _IO_ITER i;
+
+  if (stdio_needs_locking)
+    return;
+  stdio_needs_locking = 1;
+  for (i = _IO_iter_begin (); i != _IO_iter_end (); i = _IO_iter_next (i))
+    _IO_iter_file (i)->_flags2 |= _IO_FLAGS2_NEED_LOCK;
+}
+libc_hidden_def (_IO_enable_locks)
+
 void
 _IO_old_init (_IO_FILE *fp, int flags)
 {
   fp->_flags = _IO_MAGIC|flags;
   fp->_flags2 = 0;
+  if (stdio_needs_locking)
+    fp->_flags2 |= _IO_FLAGS2_NEED_LOCK;
   fp->_IO_buf_base = NULL;
   fp->_IO_buf_end = NULL;
   fp->_IO_read_base = NULL;
diff --git a/libio/getc.c b/libio/getc.c
index b58fd62308..fd66ef93cf 100644
--- a/libio/getc.c
+++ b/libio/getc.c
@@ -34,6 +34,8 @@ _IO_getc (FILE *fp)
 {
   int result;
   CHECK_FILE (fp, EOF);
+  if (!_IO_need_lock (fp))
+    return _IO_getc_unlocked (fp);
   _IO_acquire_lock (fp);
   result = _IO_getc_unlocked (fp);
   _IO_release_lock (fp);
diff --git a/libio/getchar.c b/libio/getchar.c
index 5b41595d17..d79932114e 100644
--- a/libio/getchar.c
+++ b/libio/getchar.c
@@ -33,6 +33,8 @@ int
 getchar (void)
 {
   int result;
+  if (!_IO_need_lock (_IO_stdin))
+    return _IO_getc_unlocked (_IO_stdin);
   _IO_acquire_lock (_IO_stdin);
   result = _IO_getc_unlocked (_IO_stdin);
   _IO_release_lock (_IO_stdin);
diff --git a/libio/iofopncook.c b/libio/iofopncook.c
index a08dfdaa42..982f464a68 100644
--- a/libio/iofopncook.c
+++ b/libio/iofopncook.c
@@ -172,6 +172,8 @@ _IO_cookie_init (struct _IO_cookie_file *cfile, int read_write,
   _IO_mask_flags (&cfile->__fp.file, read_write,
 		  _IO_NO_READS+_IO_NO_WRITES+_IO_IS_APPENDING);
 
+  cfile->__fp.file._flags2 |= _IO_FLAGS2_NEED_LOCK;
+
   /* We use a negative number different from -1 for _fileno to mark that
      this special stream is not associated with a real file, but still has
      to be treated as such.  */
diff --git a/libio/ioungetc.c b/libio/ioungetc.c
index 951064fa12..917cad8abb 100644
--- a/libio/ioungetc.c
+++ b/libio/ioungetc.c
@@ -33,6 +33,8 @@ _IO_ungetc (int c, _IO_FILE *fp)
   CHECK_FILE (fp, EOF);
   if (c == EOF)
     return EOF;
+  if (!_IO_need_lock (fp))
+    return _IO_sputbackc (fp, (unsigned char) c);
   _IO_acquire_lock (fp);
   result = _IO_sputbackc (fp, (unsigned char) c);
   _IO_release_lock (fp);
diff --git a/libio/libio.h b/libio/libio.h
index 518ffd8e44..14bcb92332 100644
--- a/libio/libio.h
+++ b/libio/libio.h
@@ -119,6 +119,7 @@
 # define _IO_FLAGS2_SCANF_STD 16
 # define _IO_FLAGS2_NOCLOSE 32
 # define _IO_FLAGS2_CLOEXEC 64
+# define _IO_FLAGS2_NEED_LOCK 128
 #endif
 
 /* These are "formatting flags" matching the iostream fmtflags enum values. */
@@ -451,6 +452,9 @@ extern int _IO_ftrylockfile (_IO_FILE *) __THROW;
 #define _IO_cleanup_region_end(_Doit) /**/
 #endif
 
+#define _IO_need_lock(_fp) \
+  (((_fp)->_flags2 & _IO_FLAGS2_NEED_LOCK) != 0)
+
 extern int _IO_vfscanf (_IO_FILE * __restrict, const char * __restrict,
 			_IO_va_list, int *__restrict);
 extern int _IO_vfprintf (_IO_FILE *__restrict, const char *__restrict,
diff --git a/libio/libioP.h b/libio/libioP.h
index eb93418c8d..1832b44cc7 100644
--- a/libio/libioP.h
+++ b/libio/libioP.h
@@ -444,6 +444,8 @@ extern void _IO_list_unlock (void) __THROW;
 libc_hidden_proto (_IO_list_unlock)
 extern void _IO_list_resetlock (void) __THROW;
 libc_hidden_proto (_IO_list_resetlock)
+extern void _IO_enable_locks (void) __THROW;
+libc_hidden_proto (_IO_enable_locks)
 
 /* Default jumptable functions. */
 
diff --git a/libio/putc.c b/libio/putc.c
index b591c5538b..6e1fdeef3a 100644
--- a/libio/putc.c
+++ b/libio/putc.c
@@ -25,6 +25,8 @@ _IO_putc (int c, _IO_FILE *fp)
 {
   int result;
   CHECK_FILE (fp, EOF);
+  if (!_IO_need_lock (fp))
+    return _IO_putc_unlocked (c, fp);
   _IO_acquire_lock (fp);
   result = _IO_putc_unlocked (c, fp);
   _IO_release_lock (fp);