Re: [PATCH] md: fix soft lockup during resync when sync is repeatedly skipped

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Repeatedly skipped I/O blocks cause an event storm, calling md_new_event() millions of times in a tight loop.
--

--- Patch [1]: [PATCH] md: fix soft lockup during resync when sync is repeatedly skipped ---
commit 02c063c34357030eaa8514f57d5e37328d410c0b
Author: Yunye Zhao <yunye.zhao@linux.alibaba.com>

md: fix soft lockup during resync when sync is repeatedly skipped

The md_do_sync() main loop advances io_sectors only when I/O is actually
issued. When regions are skipped, io_sectors never increases and the loop
takes the continue branch, eventually triggering a soft lockup on
non-preemptive kernels. This patch adds cond_resched() to this continue
path to prevent the soft lockup.

> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index d1465bcd86c81..e7411b0334901 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -9881,9 +9881,10 @@ void md_do_sync(struct md_thread *thread)
>  			 */
>  			md_new_event();
>  
> -		if (last_check + window > io_sectors || j == max_sectors)
> +		if (last_check + window > io_sectors || j == max_sectors) {
> +			cond_resched();
>  			continue;

[Severity: High]
This is a pre-existing issue, but can this loop in md_do_sync() cause an event
storm when I/O is repeatedly skipped?

During array recovery or resync with a sparse bitmap, io_sectors remains 0
when regions are skipped. Because last_check starts at 0, the check
(last_check + window > io_sectors) evaluates to true, causing the loop to
continue.

Since last_check is only updated after this continue statement, it remains 0
permanently across skipped regions. As a result, the preceding check for
(last_check == 0) remains true, calling md_new_event() on every skipped
iteration. This repeatedly wakes up user-space processes polling /proc/mdstat
and causes lock contention.

> -
> +		}
>  		last_check = io_sectors;
>  	repeat:

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260717062743.128189-1-yunye.zhao@linux.alibaba.com?part=1