[PATCH v2 2/8] md: don't wait for q->limits_lock in check_sb_changes()

[Jack Wang <jinpu.wang@ionos.com>]

From: Jack Wang <jinpu.wang@cloud.ionos.com>

check_sb_changes() activates a spare another node added, reached from
md_reload_sb() -> process_metadata_update() with reconfig_mutex held.
Stacking the device's limits there waits for q->limits_lock under that
mutex, which deadlocks: the lock's holder waits for the queue to drain,
and that I/O can be waiting for a superblock update needing
reconfig_mutex.

The device is already a member, so its limits are stacked.  Add it with
MDDEV_STACK_SKIP and leave them alone.

Assisted-by: LLM
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
---
 drivers/md/md.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 28fc903ffeea..e60dc2c7eb90 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -10885,7 +10885,14 @@ static void check_sb_changes(struct mddev *mddev, struct md_rdev *rdev)
 					rdev2->saved_raid_disk = -1;
 				else
 					rdev2->saved_raid_disk = role;
-				ret = remove_and_add_spares(mddev, rdev2, NULL);
+				/*
+				 * reconfig_mutex is held, so q->limits_lock
+				 * cannot be taken here.  The device is
+				 * already a member, its limits are stacked,
+				 * so add it without touching them.
+				 */
+				ret = remove_and_add_spares(mddev, rdev2,
+							    MDDEV_STACK_SKIP);
 				pr_info("Activated spare: %pg\n",
 					rdev2->bdev);
 				/* wakeup mddev->thread here, so array could
-- 
2.43.0