kthreads: fix kthread_create() vs kthread_stop() race

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@redhat.com>
Tested-by: Robert Peterson <rpeterso@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 96cff2f..9548d52 100644 (file)
--- 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;
        wake_up_process(k);
-       put_task_struct(k);
 
        /* Once it dies, reset stop ptr, gather result and we're done. */
        wait_for_completion(&kthread_stop_info.done);
        kthread_stop_info.k = NULL;
        ret = kthread_stop_info.err;
+       put_task_struct(k);
        mutex_unlock(&kthread_stop_lock);
 
        return ret;