[patch 38/48] kthreads: fix kthread_create() vs kthread_stop() race

From: Greg KH
Date: Fri Sep 04 2009 - 16:16:36 EST

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

From: Oleg Nesterov <oleg@xxxxxxxxxx>

The bug should be "accidently" fixed by recent changes in 2.6.31,
all kernels <= 2.6.30 need the fix. The problem was never noticed before,
it was found because it causes mysterious failures with GFS mount/umount.

Credits to Robert Peterson. He blaimed kthread.c from the very beginning.
But, despite my promise, I forgot to inspect the old implementation until
he did a lot of testing and reminded me. This led to huge delay in fixing
this bug.

kthread_stop() does put_task_struct(k) before it clears kthread_stop_info.k.
This means another kthread_create() can re-use this task_struct, but the
new kthread can still see kthread_should_stop() == T and exit even without
calling threadfn().

Reported-by: Robert Peterson <rpeterso@xxxxxxxxxx>
Tested-by: Robert Peterson <rpeterso@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

kernel/kthread.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -213,12 +213,12 @@ int kthread_stop(struct task_struct *k)
/* Now set kthread_should_stop() to true, and wake it up. */
kthread_stop_info.k = k;
- put_task_struct(k);

/* Once it dies, reset stop ptr, gather result and we're done. */
kthread_stop_info.k = NULL;
ret = kthread_stop_info.err;
+ put_task_struct(k);

return ret;

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