md: Close race when setting 'action' to 'idle'.

Checking ->sync_thread without holding the mddev_lock()
isn't really safe, even after flushing the workqueue which
ensures md_start_sync() has been run.

While this code is waiting for the lock, md_check_recovery could reap
the thread itself, and then start another thread (e.g. recovery might
finish, then reshape starts).  When this thread gets the lock
md_start_sync() hasn't run so it doesn't get reaped, but
MD_RECOVERY_RUNNING gets cleared.  This allows two threads to start
which leads to confusion.

So don't both if MD_RECOVERY_RUNNING isn't set, but if it is do
the flush and the test and the reap all under the mddev_lock to
avoid any race with md_check_recovery.

Signed-off-by: NeilBrown <neilb@suse.de>
Fixes: 6791875e2e53 ("md: make reconfig_mutex optional for writes to md sysfs files.")
Cc: stable@vger.kernel.org (v4.0+)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index dd59d71..8d4408b 100644 (file)
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -4217,13 +4217,14 @@ action_store(struct mddev *mddev, const char *page, size_t len)
                        set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
                else
                        clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
-               flush_workqueue(md_misc_wq);
-               if (mddev->sync_thread) {
-                       set_bit(MD_RECOVERY_INTR, &mddev->recovery);
-                       if (mddev_lock(mddev) == 0) {
+               if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
+                   mddev_lock(mddev) == 0) {
+                       flush_workqueue(md_misc_wq);
+                       if (mddev->sync_thread) {
+                               set_bit(MD_RECOVERY_INTR, &mddev->recovery);
                                md_reap_sync_thread(mddev);
-                               mddev_unlock(mddev);
                        }
+                       mddev_unlock(mddev);
                }
        } else if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) ||
                   test_bit(MD_RECOVERY_NEEDED, &mddev->recovery))