Refactor atfork handlers

Current implementation (sysdeps/nptl/fork.c) replicates the atfork
handlers list backward to invoke the child handlers after fork/clone
syscall.

The internal atfork handlers is implemented as a single-linked list
so a lock-free algorithm can be used, trading fork mulithread call
performance for some code complexity and dynamic stack allocation
(since the backwards list should not fail).

This patch refactor it to use a dynarary instead of a linked list.
It simplifies the external variables need to be exported and also
the internal atfork handler member definition.

The downside is a serialization of fork call in multithread, since to
operate on the dynarray the internal lock should be used.  However
as noted by Florian, it already acquires external locks for malloc
and libio so it is already hitting some lock contention.  Besides,
posix_spawn should be faster and more scalable to run external programs
in multithread environments.

Checked on x86_64-linux-gnu.

	* nptl/Makefile (routines): Remove unregister-atfork.
	* nptl/register-atfork.c (fork_handler_pool): Remove variable.
	(fork_handler_alloc): Remove function.
	(fork_handlers, fork_handler_init): New variables.
	(__fork_lock): Rename to atfork_lock.
	(__register_atfork, __unregister_atfork, libc_freeres_fn): Rewrite
	to use a dynamic array to add/remove atfork handlers.
	* sysdeps/nptl/fork.c (__libc_fork): Likewise.
	* sysdeps/nptl/fork.h (__fork_lock, __fork_handlers, __linkin_atfork):
	Remove declaration.
	(fork_handler): Remove next, refcntr, and need_signal member.
	(__run_fork_handler_type): New enum.
	(__run_fork_handlers): New prototype.
	* sysdeps/nptl/libc-lockP.h (__libc_atfork): Remove declaration.

1 files changed, 3 insertions, 93 deletions

diff --git a/sysdeps/nptl/fork.c b/sysdeps/nptl/fork.c
index 846fa49..f00dd64 100644
--- a/sysdeps/nptl/fork.c
+++ b/sysdeps/nptl/fork.c
@@ -48,11 +48,6 @@ pid_t
 __libc_fork (void)
 {
   pid_t pid;
-  struct used_handler
-  {
-    struct fork_handler *handler;
-    struct used_handler *next;
-  } *allp = NULL;
 
   /* Determine if we are running multiple threads.  We skip some fork
      handlers in the single-thread case, to make fork safer to use in
@@ -60,60 +55,7 @@ __libc_fork (void)
      but our current fork implementation is not.  */
   bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
 
-  /* Run all the registered preparation handlers.  In reverse order.
-     While doing this we build up a list of all the entries.  */
-  struct fork_handler *runp;
-  while ((runp = __fork_handlers) != NULL)
-    {
-      /* Make sure we read from the current RUNP pointer.  */
-      atomic_full_barrier ();
-
-      unsigned int oldval = runp->refcntr;
-
-      if (oldval == 0)
-	/* This means some other thread removed the list just after
-	   the pointer has been loaded.  Try again.  Either the list
-	   is empty or we can retry it.  */
-	continue;
-
-      /* Bump the reference counter.  */
-      if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
-						oldval + 1, oldval))
-	/* The value changed, try again.  */
-	continue;
-
-      /* We bumped the reference counter for the first entry in the
-	 list.  That means that none of the following entries will
-	 just go away.  The unloading code works in the order of the
-	 list.
-
-	 While executing the registered handlers we are building a
-	 list of all the entries so that we can go backward later on.  */
-      while (1)
-	{
-	  /* Execute the handler if there is one.  */
-	  if (runp->prepare_handler != NULL)
-	    runp->prepare_handler ();
-
-	  /* Create a new element for the list.  */
-	  struct used_handler *newp
-	    = (struct used_handler *) alloca (sizeof (*newp));
-	  newp->handler = runp;
-	  newp->next = allp;
-	  allp = newp;
-
-	  /* Advance to the next handler.  */
-	  runp = runp->next;
-	  if (runp == NULL)
-	    break;
-
-	  /* Bump the reference counter for the next entry.  */
-	  atomic_increment (&runp->refcntr);
-	}
-
-      /* We are done.  */
-      break;
-    }
+  __run_fork_handlers (atfork_run_prepare);
 
   /* If we are not running multiple threads, we do not have to
      preserve lock state.  If fork runs from a signal handler, only
@@ -198,29 +140,7 @@ __libc_fork (void)
       __rtld_lock_initialize (GL(dl_load_lock));
 
       /* Run the handlers registered for the child.  */
-      while (allp != NULL)
-	{
-	  if (allp->handler->child_handler != NULL)
-	    allp->handler->child_handler ();
-
-	  /* Note that we do not have to wake any possible waiter.
-	     This is the only thread in the new process.  The count
-	     may have been bumped up by other threads doing a fork.
-	     We reset it to 1, to avoid waiting for non-existing
-	     thread(s) to release the count.  */
-	  allp->handler->refcntr = 1;
-
-	  /* XXX We could at this point look through the object pool
-	     and mark all objects not on the __fork_handlers list as
-	     unused.  This is necessary in case the fork() happened
-	     while another thread called dlclose() and that call had
-	     to create a new list.  */
-
-	  allp = allp->next;
-	}
-
-      /* Initialize the fork lock.  */
-      __fork_lock = LLL_LOCK_INITIALIZER;
+      __run_fork_handlers (atfork_run_child);
     }
   else
     {
@@ -235,17 +155,7 @@ __libc_fork (void)
 	}
 
       /* Run the handlers registered for the parent.  */
-      while (allp != NULL)
-	{
-	  if (allp->handler->parent_handler != NULL)
-	    allp->handler->parent_handler ();
-
-	  if (atomic_decrement_and_test (&allp->handler->refcntr)
-	      && allp->handler->need_signal)
-	    futex_wake (&allp->handler->refcntr, 1, FUTEX_PRIVATE);
-
-	  allp = allp->next;
-	}
+      __run_fork_handlers (atfork_run_parent);
     }
 
   return pid;