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+)

1 files changed, 6 insertions, 5 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index dd59d71ade2f..8d4408baa428 100644
--- 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))