[PATCH 02/10] [PATCH] Fix fs/exec.c:788 (de_thread()) BUG_ON

Chris Wright
2005-09-30T02:23:31Z

[-- Attachment #1: fix-de_thread-BUG_ON.patch --]
[-- Type: text/plain, Size: 2012 bytes --]

-stable review patch. If anyone has any objections, please let us know.
------------------

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>
Signed-off-by: Chris Wright <chrisw@osdl.org>
---
fs/exec.c | 5 ++---
1 files changed, 2 insertions(+), 3 deletions(-)

Index: linux-2.6.13.y/fs/exec.c
===================================================================
--- linux-2.6.13.y.orig/fs/exec.c
+++ linux-2.6.13.y/fs/exec.c
@@ -745,8 +745,8 @@ static inline int de_thread(struct task_
}

/*
- * 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;
}

--