posix: Consolidate fork implementation

The Linux nptl implementation is used as base for generic fork
implementation to handle the internal locks and mutexes.  The
system specific bits are moved a new internal _Fork symbol.

(This new implementation will be used to provide a async-signal-safe
_Fork now that POSIX has clarified that fork might not be
async-signal-safe [1]).

For Hurd it means that the __nss_database_fork_prepare_parent and
__nss_database_fork_subprocess will be run in a slight different
order.

[1] https://austingroupbugs.net/view.php?id=62

1 files changed, 148 insertions, 0 deletions

diff --git a/sysdeps/nptl/fork.h b/sysdeps/nptl/fork.h
new file mode 100644
index 0000000000..3134d7ab94
--- /dev/null
+++ b/sysdeps/nptl/fork.h
@@ -0,0 +1,148 @@
+/* System specific fork hooks.  Linux version.
+   Copyright (C) 2021 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 _FORK_H
+#define _FORK_H
+
+#include <assert.h>
+#include <ldsodefs.h>
+#include <list.h>
+#include <pthreadP.h>
+#include <sysdep.h>
+
+static inline void
+fork_system_setup (void)
+{
+  /* See __pthread_once.  */
+  __fork_generation += __PTHREAD_ONCE_FORK_GEN_INCR;
+}
+
+static void
+fork_system_setup_after_fork (void)
+{
+  /* There is one thread running.  */
+  __nptl_nthreads = 1;
+
+  /* Initialize thread library locks.  */
+  GL (dl_stack_cache_lock) = LLL_LOCK_INITIALIZER;
+  __default_pthread_attr_lock = LLL_LOCK_INITIALIZER;
+}
+
+/* In case of a fork() call the memory allocation in the child will be
+   the same but only one thread is running.  All stacks except that of
+   the one running thread are not used anymore.  We have to recycle
+   them.  */
+static void
+reclaim_stacks (void)
+{
+  struct pthread *self = (struct pthread *) THREAD_SELF;
+
+  /* No locking necessary.  The caller is the only stack in use.  But
+     we have to be aware that we might have interrupted a list
+     operation.  */
+
+  if (GL (dl_in_flight_stack) != 0)
+    {
+      bool add_p = GL (dl_in_flight_stack) & 1;
+      list_t *elem = (list_t *) (GL (dl_in_flight_stack) & ~(uintptr_t) 1);
+
+      if (add_p)
+	{
+	  /* We always add at the beginning of the list.  So in this case we
+	     only need to check the beginning of these lists to see if the
+	     pointers at the head of the list are inconsistent.  */
+	  list_t *l = NULL;
+
+	  if (GL (dl_stack_used).next->prev != &GL (dl_stack_used))
+	    l = &GL (dl_stack_used);
+	  else if (GL (dl_stack_cache).next->prev != &GL (dl_stack_cache))
+	    l = &GL (dl_stack_cache);
+
+	  if (l != NULL)
+	    {
+	      assert (l->next->prev == elem);
+	      elem->next = l->next;
+	      elem->prev = l;
+	      l->next = elem;
+	    }
+	}
+      else
+	{
+	  /* We can simply always replay the delete operation.  */
+	  elem->next->prev = elem->prev;
+	  elem->prev->next = elem->next;
+	}
+
+      GL (dl_in_flight_stack) = 0;
+    }
+
+  /* Mark all stacks except the still running one as free.  */
+  list_t *runp;
+  list_for_each (runp, &GL (dl_stack_used))
+    {
+      struct pthread *curp = list_entry (runp, struct pthread, list);
+      if (curp != self)
+	{
+	  /* This marks the stack as free.  */
+	  curp->tid = 0;
+
+	  /* Account for the size of the stack.  */
+	  GL (dl_stack_cache_actsize) += curp->stackblock_size;
+
+	  if (curp->specific_used)
+	    {
+	      /* Clear the thread-specific data.  */
+	      memset (curp->specific_1stblock, '\0',
+		      sizeof (curp->specific_1stblock));
+
+	      curp->specific_used = false;
+
+	      for (size_t cnt = 1; cnt < PTHREAD_KEY_1STLEVEL_SIZE; ++cnt)
+		if (curp->specific[cnt] != NULL)
+		  {
+		    memset (curp->specific[cnt], '\0',
+			    sizeof (curp->specific_1stblock));
+
+		    /* We have allocated the block which we do not
+		       free here so re-set the bit.  */
+		    curp->specific_used = true;
+		  }
+	    }
+	}
+    }
+
+  /* Add the stack of all running threads to the cache.  */
+  list_splice (&GL (dl_stack_used), &GL (dl_stack_cache));
+
+  /* Remove the entry for the current thread to from the cache list
+     and add it to the list of running threads.  Which of the two
+     lists is decided by the user_stack flag.  */
+  list_del (&self->list);
+
+  /* Re-initialize the lists for all the threads.  */
+  INIT_LIST_HEAD (&GL (dl_stack_used));
+  INIT_LIST_HEAD (&GL (dl_stack_user));
+
+  if (__glibc_unlikely (THREAD_GETMEM (self, user_stack)))
+    list_add (&self->list, &GL (dl_stack_user));
+  else
+    list_add (&self->list, &GL (dl_stack_used));
+}
+
+
+#endif