malloc: Run fork handler as late as possible [BZ #19431]

Previously, a thread M invoking fork would acquire locks in this order:

  (M1) malloc arena locks (in the registered fork handler)
  (M2) libio list lock

A thread F invoking flush (NULL) would acquire locks in this order:

  (F1) libio list lock
  (F2) individual _IO_FILE locks

A thread G running getdelim would use this order:

  (G1) _IO_FILE lock
  (G2) malloc arena lock

After executing (M1), (F1), (G1), none of the threads can make progress.

This commit changes the fork lock order to:

  (M'1) libio list lock
  (M'2) malloc arena locks

It explicitly encodes the lock order in the implementations of fork,
and does not rely on the registration order, thus avoiding the deadlock.

1 files changed, 13 insertions, 0 deletions

diff --git a/sysdeps/mach/hurd/fork.c b/sysdeps/mach/hurd/fork.c
index ad09fd7c41..2e8b59e7c0 100644
--- a/sysdeps/mach/hurd/fork.c
+++ b/sysdeps/mach/hurd/fork.c
@@ -26,6 +26,7 @@
 #include <assert.h>
 #include "hurdmalloc.h"		/* XXX */
 #include <tls.h>
+#include <malloc/malloc-internal.h>
 
 #undef __fork
 
@@ -107,6 +108,12 @@ __fork (void)
       /* Run things that prepare for forking before we create the task.  */
       RUN_HOOK (_hurd_fork_prepare_hook, ());
 
+      /* Acquire malloc locks.  This needs to come last because fork
+	 handlers may use malloc, and the libio list lock has an
+	 indirect malloc dependency as well (via the getdelim
+	 function).  */
+      __malloc_fork_lock_parent ();
+
       /* Lock things that want to be locked before we fork.  */
       {
 	void *const *p;
@@ -604,6 +611,9 @@ __fork (void)
 			   nthreads * sizeof (*threads));
 	}
 
+      /* Release malloc locks.  */
+      __malloc_fork_unlock_parent ();
+
       /* Run things that want to run in the parent to restore it to
 	 normality.  Usually prepare hooks and parent hooks are
 	 symmetrical: the prepare hook arrests state in some way for the
@@ -655,6 +665,9 @@ __fork (void)
       /* Forking clears the trace flag.  */
       __sigemptyset (&_hurdsig_traced);
 
+      /* Release malloc locks.  */
+      __malloc_fork_unlock_child ();
+
       /* Run things that want to run in the child task to set up.  */
       RUN_HOOK (_hurd_fork_child_hook, ());