[PATCH] md: close a small race in md thread deregistration

There is a tiny race when de-registering an MD thread, in that the thread
could disappear before it is set a SIGKILL, causing send_sig to have
problems.  

This is most easily closed by holding tasklist_lock between enabling the
thread to exit (setting ->run to NULL) and telling it to exit.

(akpm: ick.  Needs to use kthread API and stop using signals)

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 12 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index aa72c88a024..44a16496554 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2840,16 +2840,6 @@ mdk_thread_t *md_register_thread(void (*run) (mddev_t *), mddev_t *mddev,
         return thread;
 }
 
-static void md_interrupt_thread(mdk_thread_t *thread)
-{
-        if (!thread->tsk) {
-                MD_BUG();
-                return;
-        }
-        dprintk("interrupting MD-thread pid %d\n", thread->tsk->pid);
-        send_sig(SIGKILL, thread->tsk, 1);
-}
-
 void md_unregister_thread(mdk_thread_t *thread)
 {
         struct completion event;
@@ -2857,9 +2847,15 @@ void md_unregister_thread(mdk_thread_t *thread)
         init_completion(&event);
 
         thread->event = &event;
+
+        /* As soon as ->run is set to NULL, the task could disappear,
+         * so we need to hold tasklist_lock until we have sent the signal
+         */
+        dprintk("interrupting MD-thread pid %d\n", thread->tsk->pid);
+        read_lock(&tasklist_lock);
         thread->run = NULL;
-        thread->name = NULL;
-        md_interrupt_thread(thread);
+        send_sig(SIGKILL, thread->tsk, 1);
+        read_unlock(&tasklist_lock);
         wait_for_completion(&event);
         kfree(thread);
 }