linux-kernel - [PATCH] md/raid5: fix deadlock that raid5d() wait for itself to clear MD_SB_CHANGE

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Date: Fri, 22 Mar 2024 16:10:05 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: song@...nel.org,
	logang@...tatee.com,
	dan@...m.net,
	junxiao.bi@...cle.com,
	xni@...hat.com
Cc: linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yukuai3@...wei.com,
	yukuai1@...weicloud.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com
Subject: [PATCH] md/raid5: fix deadlock that raid5d() wait for itself to clear MD_SB_CHANGE_PENDING

From: Yu Kuai <yukuai3@...wei.com>

Xiao reported that lvm2 test lvconvert-raid-takeover.sh can hang with
small possibility, the root cause is exactly the same as commit
bed9e27baf52 ("Revert "md/raid5: Wait for MD_SB_CHANGE_PENDING in raid5d"")

However, Dan reported another hang after that, and junxiao investigated
the problem and found out that this is caused by plugged bio can't issue
from raid5d().

Current implementation in raid5d() has a weird dependence:

1) md_check_recovery() from raid5d() must hold 'reconfig_mutex' to clear
   MD_SB_CHANGE_PENDING;
2) raid5d() handles IO in a deadloop, until all IO are issued;
3) IO from raid5d() must wait for MD_SB_CHANGE_PENDING to be cleared;

This behaviour is introduce before v2.6, and for consequence, if other
context hold 'reconfig_mutex', and md_check_recovery() can't update
super_block, then raid5d() will waste one cpu 100% by the deadloop, until
'reconfig_mutex' is released.

Refer to the implementation from raid1 and raid10, fix this problem by
skipping issue IO if MD_SB_CHANGE_PENDING is still set after
md_check_recovery(), daemon thread will be woken up when 'reconfig_mutex'
is released. Meanwhile, the hang problem will be fixed as well.

Fixes: 5e2cf333b7bd ("md/raid5: Wait for MD_SB_CHANGE_PENDING in raid5d")
Reported-and-tested-by: Dan Moulding <dan@...m.net>
Closes: https://lore.kernel.org/all/20240123005700.9302-1-dan@danm.net/
Investigated-by: Junxiao Bi <junxiao.bi@...cle.com>
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 drivers/md/raid5.c | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index d874abfc1836..2bd1ce9b3922 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -36,7 +36,6 @@
  */
 
 #include <linux/blkdev.h>
-#include <linux/delay.h>
 #include <linux/kthread.h>
 #include <linux/raid/pq.h>
 #include <linux/async_tx.h>
@@ -6734,6 +6733,9 @@ static void raid5d(struct md_thread *thread)
 		int batch_size, released;
 		unsigned int offset;
 
+		if (test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
+			break;
+
 		released = release_stripe_list(conf, conf->temp_inactive_list);
 		if (released)
 			clear_bit(R5_DID_ALLOC, &conf->cache_state);
@@ -6770,18 +6772,7 @@ static void raid5d(struct md_thread *thread)
 			spin_unlock_irq(&conf->device_lock);
 			md_check_recovery(mddev);
 			spin_lock_irq(&conf->device_lock);
-
-			/*
-			 * Waiting on MD_SB_CHANGE_PENDING below may deadlock
-			 * seeing md_check_recovery() is needed to clear
-			 * the flag when using mdmon.
-			 */
-			continue;
 		}
-
-		wait_event_lock_irq(mddev->sb_wait,
-			!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags),
-			conf->device_lock);
 	}
 	pr_debug("%d stripes handled\n", handled);
 
-- 
2.39.2