Re:Re: [PATCH] md/raid5: serialize plug list add with device_lock

[李佑鸿]

Hi Kuai,












At 2026-09-05 11:17:58, "yu kuai" <yukuai@xxxxxxx> wrote:
>Hi,
>
>在 2026/9/2 17:53, Li Youhong 写道:
>> From: Li Youhong <liyouhong@xxxxxxxxxx>
>>
>> raid5_unplug() can spin forever under conf->device_lock when
>> raid5_plug_cb.list still points at a stripe whose sh->lru has
>> already been reinitialized (self-looped). That disables IRQs on
>> the holder CPU and causes multi-CPU hard lockups on waiters of
>> the same lock.
>>
>> This happens because release_stripe_plug() sets
>> STRIPE_ON_UNPLUG_LIST and list_add_tail(sh->lru) without
>> device_lock, while do_release_stripe() may concurrently move the
>> same lru onto handle/inactive when the last reference drops.
>>
>> Note: a 2020 proposal tried extra refs / checking
>> STRIPE_ON_UNPLUG_LIST in do_release_stripe() without serializing
>> the plug enqueue:
>> https://lore.kernel.org/linux-raid/20200108163023.9301-1-guoqing.jiang@xxxxxxxxxxxxxxx/
>> That still leaves a TOCTOU window where the bit is clear during
>> the check and set afterwards, allowing two list_add on the same
>> lru. It was not merged.
>>
>> Serialize the bit update and list_add with device_lock. If
>> do_release_stripe() still sees STRIPE_ON_UNPLUG_LIST, restore the
>> reference and let raid5_unplug() own the final release.
>>
>> Observed on production 9-disk NVMe RAID5 under MySQL AIO
>> (io_submit -> blk_finish_plug -> raid5_unplug).
>>
>> Fixes: 8811b5968f62 ("raid5: make_request use batch stripe release")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Li Youhong <liyouhong@xxxxxxxxxx>
>> ---
>>   drivers/md/raid5.c | 33 ++++++++++++++++++++++++++++++---
>>   1 file changed, 30 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>> index b91545ce090d..c9197b63b1c4 100644
>> --- a/drivers/md/raid5.c
>> +++ b/drivers/md/raid5.c
>> @@ -229,6 +229,17 @@ static void do_release_stripe(struct r5conf *conf, struct stripe_head *sh,
>>   	int i;
>>   	int injournal = 0;	/* number of date pages with R5_InJournal */
>>   
>> +	/*
>> +	 * Stripe is owned by release_stripe_plug()'s cb->list. A concurrent
>> +	 * last-ref release can reach here after the stripe was queued for
>> +	 * unplug (lru may already be non-empty). Do not re-add lru elsewhere;
>> +	 * restore the reference and let raid5_unplug() finish the release.
>> +	 */
>> +	if (test_bit(STRIPE_ON_UNPLUG_LIST, &sh->state)) {
>> +		atomic_inc(&sh->count);
>> +		return;
>> +	}
>> +
>>   	BUG_ON(!list_empty(&sh->lru));
>>   	BUG_ON(atomic_read(&conf->active_stripes)==0);
>>   
>> @@ -5760,6 +5771,9 @@ static void release_stripe_plug(struct mddev *mddev,
>>   		raid5_unplug, mddev,
>>   		sizeof(struct raid5_plug_cb));
>>   	struct raid5_plug_cb *cb;
>> +	struct r5conf *conf = mddev->private;
>> +	unsigned long flags;
>> +	bool queued = false;
>>   
>>   	if (!blk_cb) {
>>   		raid5_release_stripe(sh);
>> @@ -5775,9 +5789,22 @@ static void release_stripe_plug(struct mddev *mddev,
>>   			INIT_LIST_HEAD(cb->temp_inactive_list + i);
>>   	}
>>   
>> -	if (!test_and_set_bit(STRIPE_ON_UNPLUG_LIST, &sh->state))
>> -		list_add_tail(&sh->lru, &cb->list);
>> -	else
>> +	/*
>> +	 * Serialize with do_release_stripe() on device_lock so sh->lru cannot
>> +	 * be added to handle/inactive and cb->list at the same time.
>> +	 */
>> +	spin_lock_irqsave(&conf->device_lock, flags);
>> +	if (!test_and_set_bit(STRIPE_ON_UNPLUG_LIST, &sh->state)) {
>> +		if (unlikely(!list_empty(&sh->lru))) {
>> +			clear_bit(STRIPE_ON_UNPLUG_LIST, &sh->state);
>> +		} else {
>> +			list_add_tail(&sh->lru, &cb->list);
>> +			queued = true;
>> +		}
>> +	}
>> +	spin_unlock_irqrestore(&conf->device_lock, flags);
>
>Do you run some performance test on this patch? I believe this is from IO hot path,
>a spinlock is not acceptable. I know there is already some spinlock for raid5, but
>I'd like not to introduce new lock contention.
>

>Can this problem be fixed by a new llist in stripe_head?


I agree that adding device_lock on this path can bring some
overhead; I will follow up with performance numbers.


Regarding the alternative: did you mean adding a dedicated
llist_node in stripe_head for the plug/unplug path (instead of
reusing sh->lru), so that release_stripe_plug() and
do_release_stripe() no longer share the same list node?


If we still queue the plug list via sh->lru, switching that list
to an llist would not remove the race, because do_release_stripe()
can still list_add the same lru onto handle/inactive concurrently.
A separate node would avoid that, but unplug would then need to
move stripes from the plug llist onto the existing lru-based
release path, which is a larger change.


Please let me know if that matches what you had in mind.


Thanks,
Li Youhong>
>> +
>> +	if (!queued)
>>   		raid5_release_stripe(sh);
>>   }
>>   
>
>-- 
>Thanks,
>Kuai