Re: [PATCH] md/raid10: fix missing wakeup in wait_barrier_nolock

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Lockless read of `conf->array_freeze_pending` lacks `READ_ONCE()`, violating kernel concurrency guidelines and potentially triggering KCSAN warnings.
--

--- Patch [1]: [PATCH] md/raid10: fix missing wakeup in wait_barrier_nolock ---
commit 3470079c0f4271a77d161b3a70575176ea1d322b
Author: Zizhi Wo <[email protected]>

md/raid10: fix missing wakeup in wait_barrier_nolock

This commit fixes a hung task issue in RAID10 by ensuring that
wait_barrier_nolock() properly wakes up a pending freeze_array(). It adds a
wake-up condition for conf->array_freeze_pending when the pending counter is
decremented.

> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 0a3cfdd3f5df8..fad634a444f89 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
[ ... ]
> @@ -996,7 +996,8 @@ static bool wait_barrier_nolock(struct r10conf *conf)
>  	if (!read_seqretry(&conf->resync_lock, seq))
>  		return true;
>  
> -	if (atomic_dec_and_test(&conf->nr_pending))
> +	if (atomic_dec_and_test(&conf->nr_pending) ||
> +	    conf->array_freeze_pending)

[Severity: Low]
Does this lockless read of conf->array_freeze_pending need a READ_ONCE()
wrapper?

Since freeze_array() modifies this variable under the resync_lock seqlock,
reading it here locklessly might trigger KCSAN data race warnings or be
subject to compiler optimizations. 

While atomic_dec_and_test() provides a memory barrier, kernel concurrency
guidelines generally require READ_ONCE() for plain lockless reads of shared
variables to prevent compiler optimizations.

It looks like this pattern was copied from allow_barrier(), which also has a
lockless read without READ_ONCE():

drivers/md/raid10.c:allow_barrier() {
    ...
    if ((atomic_dec_and_test(&conf->nr_pending)) ||
        (conf->array_freeze_pending))
        wake_up_barrier(conf);
    ...
}

Should both of these locations be updated to use
READ_ONCE(conf->array_freeze_pending)?

>  		wake_up_barrier(conf);
>  
>  	return false;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1