md: protect md_unregister_thread from reentrancy

Generally, the md_unregister_thread is called with reconfig_mutex, but
raid_message in dm-raid doesn't hold reconfig_mutex to unregister thread,
so md_unregister_thread can be called simulitaneously from two call sites
in theory.

Then after previous commit which remove the protection of reconfig_mutex
for md_unregister_thread completely, the potential issue could be worse
than before.

Let's take pers_lock at the beginning of function to ensure reentrancy.

Reported-by: Donald Buczek <buczek@molgen.mpg.de>
Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <song@kernel.org>

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 1958a14..53787a3 100644 (file)
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -7963,17 +7963,22 @@ EXPORT_SYMBOL(md_register_thread);
 
 void md_unregister_thread(struct md_thread **threadp)
 {
-       struct md_thread *thread = *threadp;
-       if (!thread)
-               return;
-       pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
-       /* Locking ensures that mddev_unlock does not wake_up a
+       struct md_thread *thread;
+
+       /*
+        * Locking ensures that mddev_unlock does not wake_up a
         * non-existent thread
         */
        spin_lock(&pers_lock);
+       thread = *threadp;
+       if (!thread) {
+               spin_unlock(&pers_lock);
+               return;
+       }
        *threadp = NULL;
        spin_unlock(&pers_lock);
 
+       pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
        kthread_stop(thread->tsk);
        kfree(thread);
 }