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

["yu kuai" <yukuai@fygo.io>]

Hi,

在 2026/9/7 11:24, 李佑鸿 写道:
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> Hi Kuai,
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> At 2026-09-05 11:17:58, "yu kuai" <yukuai@fygo.io> wrote:
>> Hi,
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>> 在 2026/9/2 17:53, Li Youhong 写道:
>>> From: Li Youhong <liyouhong@kylinos.cn>
>>>
>>> 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@cloud.ionos.com/
>>> 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@vger.kernel.org
>>> Signed-off-by: Li Youhong <liyouhong@kylinos.cn>
>>> ---
>>>    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?
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> I agree that adding device_lock on this path can bring some
> overhead; I will follow up with performance numbers.
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> 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?
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> 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.

I mean a new list for unplug in sh like unplug_llist. And I think it's
fine to use the llist directly for unplug, there is no need to move
sh from unplug llist to lru list first.

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> Please let me know if that matches what you had in mind.
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> Thanks,
> Li Youhong>
>>> +
>>> +	if (!queued)
>>>    		raid5_release_stripe(sh);
>>>    }
>>>    
>> -- 
>> Thanks,
>> Kuai

-- 
Thanks,
Kuai