linux-kernel - [PATCH 5.10 050/530] md: md_open returns -EBUSY when entering racing area

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Date:   Wed, 12 May 2021 16:42:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org
Subject: [PATCH 5.10 050/530] md: md_open returns -EBUSY when entering racing area

From: Zhao Heming <heming.zhao@...e.com>

commit 6a4db2a60306eb65bfb14ccc9fde035b74a4b4e7 upstream.

commit d3374825ce57 ("md: make devices disappear when they are no longer
needed.") introduced protection between mddev creating & removing. The
md_open shouldn't create mddev when all_mddevs list doesn't contain
mddev. With currently code logic, there will be very easy to trigger
soft lockup in non-preempt env.

This patch changes md_open returning from -ERESTARTSYS to -EBUSY, which
will break the infinitely retry when md_open enter racing area.

This patch is partly fix soft lockup issue, full fix needs mddev_find
is split into two functions: mddev_find & mddev_find_or_alloc. And
md_open should call new mddev_find (it only does searching job).

For more detail, please refer with Christoph's "split mddev_find" patch
in later commits.

---
 drivers/md/md.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -7857,8 +7857,7 @@ static int md_open(struct block_device *
 		/* Wait until bdev->bd_disk is definitely gone */
 		if (work_pending(&mddev->del_work))
 			flush_workqueue(md_misc_wq);
-		/* Then retry the open from the top */
-		return -ERESTARTSYS;
+		return -EBUSY;
 	}
 	BUG_ON(mddev != bdev->bd_disk->private_data);