From 21e0958ec9684e76e32f822c5e611a7d7ea0a5ba Mon Sep 17 00:00:00 2001
From: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Date: Sat, 4 Apr 2020 23:57:07 +0200
Subject: [PATCH] md: add checkings before flush md_misc_wq
Git-commit: 21e0958ec9684e76e32f822c5e611a7d7ea0a5ba
Patch-mainline: v5.8-rc1
References: jsc#SLE-13984
Coly reported possible circular locking dependencyi with LOCKDEP enabled,
quote the below info from the detailed report [1].
[ 1607.673903] Chain exists of:
[ 1607.673903] kn->count#256 --> (wq_completion)md_misc -->
(work_completion)(&rdev->del_work)
[ 1607.673903]
[ 1607.827946] Possible unsafe locking scenario:
[ 1607.827946]
[ 1607.898780] CPU0 CPU1
[ 1607.952980] ---- ----
[ 1608.007173] lock((work_completion)(&rdev->del_work));
[ 1608.069690] lock((wq_completion)md_misc);
[ 1608.149887] lock((work_completion)(&rdev->del_work));
[ 1608.242563] lock(kn->count#256);
[ 1608.283238]
[ 1608.283238] *** DEADLOCK ***
[ 1608.283238]
[ 1608.354078] 2 locks held by kworker/5:0/843:
[ 1608.405152] #0: ffff8889eecc9948 ((wq_completion)md_misc){+.+.}, at:
process_one_work+0x42b/0xb30
[ 1608.512399] #1: ffff888a1d3b7e10
((work_completion)(&rdev->del_work)){+.+.}, at: process_one_work+0x42b/0xb30
[ 1608.632130]
Since works (rdev->del_work and mddev->del_work) are queued in md_misc_wq,
then lockdep_map lock is held if either of them are running, then both of
them try to hold kernfs lock by call kobject_del. Then if new_dev_store
or array_state_store are triggered by write to the related sysfs node, so
the write operation gets kernfs lock, but need the lockdep_map because all
of them would trigger flush_workqueue(md_misc_wq) finally, then the same
lockdep_map lock is needed.
To suppress the lockdep warnning, we should flush the workqueue in case the
related work is pending. And several works are attached to md_misc_wq, so
we need to check which work should be checked:
1. for __md_stop_writes, the purpose of call flush workqueue is ensure sync
thread is started if it was starting, so check mddev->del_work is pending
or not since md_start_sync is attached to mddev->del_work.
2. __md_stop flushes md_misc_wq to ensure event_work is done, check the
event_work is enough. Assume raid_{ctr,dtr} -> md_stop -> __md_stop doesn't
need the kernfs lock.
3. both new_dev_store (holds kernfs lock) and ADD_NEW_DISK ioctl (holds the
bdev->bd_mutex) call flush_workqueue to ensure md_delayed_delete has
completed, this case will be handled in next patch.
4. md_open flushes workqueue to ensure the previous md is disappeared, but
it holds bdev->bd_mutex then try to flush workqueue, so it is better to
check mddev->del_work as well to avoid potential lock issue, this will be
done in another patch.
[1]: https://marc.info/?l=linux-raid&m=158518958031584&w=2
Cc: Coly Li <colyli@suse.de>
Reported-by: Coly Li <colyli@suse.de>
Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Coly Li <colyli@suse.de>
---
drivers/md/md.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 271e8a587354..76c32ca6eee8 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -6147,7 +6147,8 @@ static void md_clean(struct mddev *mddev)
static void __md_stop_writes(struct mddev *mddev)
{
set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
- flush_workqueue(md_misc_wq);
+ if (work_pending(&mddev->del_work))
+ flush_workqueue(md_misc_wq);
if (mddev->sync_thread) {
set_bit(MD_RECOVERY_INTR, &mddev->recovery);
md_reap_sync_thread(mddev);
@@ -6200,7 +6201,8 @@ static void __md_stop(struct mddev *mddev)
md_bitmap_destroy(mddev);
mddev_detach(mddev);
/* Ensure ->event_work is done */
- flush_workqueue(md_misc_wq);
+ if (mddev->event_work.func)
+ flush_workqueue(md_misc_wq);
spin_lock(&mddev->lock);
mddev->pers = NULL;
spin_unlock(&mddev->lock);
--
2.26.2