[PATCH] Fix fs/exec.c:788 (de_thread()) BUG_ON
Alexander Nyberg [Wed, 14 Sep 2005 16:54:06 +0000 (18:54 +0200)]
It turns out that the BUG_ON() in fs/exec.c: de_thread() is unreliable
and can trigger due to the test itself being racy.

de_thread() does
  while (atomic_read(&sig->count) > count) {
}
.....
.....
BUG_ON(!thread_group_empty(current));

but release_task does
write_lock_irq(&tasklist_lock)
__exit_signal
(this is where atomic_dec(&sig->count) is run)
__exit_sighand
__unhash_process
takes write lock on tasklist_lock
remove itself out of PIDTYPE_TGID list
write_unlock_irq(&tasklist_lock)

so there's a clear (although small) window between the
atomic_dec(&sig->count) and the actual PIDTYPE_TGID unhashing of the
thread.

And actually there is no need for all threads to have exited at this
point, so we simply kill the BUG_ON.

Big thanks to Marc Lehmann who provided the test-case.

Fixes Bug 5170 (http://bugme.osdl.org/show_bug.cgi?id=5170)

Signed-off-by: Alexander Nyberg <alexn@telia.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Andrew Morton <akpm@osdl.org>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/exec.c

index 14dd039..6fae59a 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -745,8 +745,8 @@ static inline int de_thread(struct task_struct *tsk)
         }
 
        /*
-        * Now there are really no other threads at all,
-        * so it's safe to stop telling them to kill themselves.
+        * There may be one thread left which is just exiting,
+        * but it's safe to stop telling the group to kill themselves.
         */
        sig->flags = 0;
 
@@ -785,7 +785,6 @@ no_thread_group:
                        kmem_cache_free(sighand_cachep, oldsighand);
        }
 
-       BUG_ON(!thread_group_empty(current));
        BUG_ON(!thread_group_leader(current));
        return 0;
 }