Re: [PATCH V2 3/3] block: model freeze & enter queue as lock for supporting lockdep

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 12.11.2024 11:15, Ming Lei wrote:
> On Tue, Nov 12, 2024 at 09:36:40AM +0100, Marek Szyprowski wrote:
>> On 29.10.2024 16:58, Ming Lei wrote:
>>> On Tue, Oct 29, 2024 at 12:13:35PM +0100, Marek Szyprowski wrote:
>>>> On 25.10.2024 02:37, Ming Lei wrote:
>>>>> Recently we got several deadlock report[1][2][3] caused by
>>>>> blk_mq_freeze_queue and blk_enter_queue().
>>>>>
>>>>> Turns out the two are just like acquiring read/write lock, so model them
>>>>> as read/write lock for supporting lockdep:
>>>>>
>>>>> 1) model q->q_usage_counter as two locks(io and queue lock)
>>>>>
>>>>> - queue lock covers sync with blk_enter_queue()
>>>>>
>>>>> - io lock covers sync with bio_enter_queue()
>>>>>
>>>>> 2) make the lockdep class/key as per-queue:
>>>>>
>>>>> - different subsystem has very different lock use pattern, shared lock
>>>>>     class causes false positive easily
>>>>>
>>>>> - freeze_queue degrades to no lock in case that disk state becomes DEAD
>>>>>      because bio_enter_queue() won't be blocked any more
>>>>>
>>>>> - freeze_queue degrades to no lock in case that request queue becomes dying
>>>>>      because blk_enter_queue() won't be blocked any more
>>>>>
>>>>> 3) model blk_mq_freeze_queue() as acquire_exclusive & try_lock
>>>>> - it is exclusive lock, so dependency with blk_enter_queue() is covered
>>>>>
>>>>> - it is trylock because blk_mq_freeze_queue() are allowed to run
>>>>>      concurrently
>>>>>
>>>>> 4) model blk_enter_queue() & bio_enter_queue() as acquire_read()
>>>>> - nested blk_enter_queue() are allowed
>>>>>
>>>>> - dependency with blk_mq_freeze_queue() is covered
>>>>>
>>>>> - blk_queue_exit() is often called from other contexts(such as irq), and
>>>>> it can't be annotated as lock_release(), so simply do it in
>>>>> blk_enter_queue(), this way still covered cases as many as possible
>>>>>
>>>>> With lockdep support, such kind of reports may be reported asap and
>>>>> needn't wait until the real deadlock is triggered.
>>>>>
>>>>> For example, lockdep report can be triggered in the report[3] with this
>>>>> patch applied.
>>>>>
>>>>> [1] occasional block layer hang when setting 'echo noop > /sys/block/sda/queue/scheduler'
>>>>> https://bugzilla.kernel.org/show_bug.cgi?id=219166
>>>>>
>>>>> [2] del_gendisk() vs blk_queue_enter() race condition
>>>>> https://lore.kernel.org/linux-block/20241003085610.GK11458@google.com/
>>>>>
>>>>> [3] queue_freeze & queue_enter deadlock in scsi
>>>>> https://lore.kernel.org/linux-block/ZxG38G9BuFdBpBHZ@fedora/T/#u
>>>>>
>>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>>>>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>>>> This patch landed yesterday in linux-next as commit f1be1788a32e
>>>> ("block: model freeze & enter queue as lock for supporting lockdep").
>>>> In my tests I found that it introduces the following 2 lockdep warnings:
>>>>
>>>>> ...
>>>>
>>>> 2. On QEMU's ARM64 virt machine, observed during system suspend/resume
>>>> cycle:
>>>>
>>>> # time rtcwake -s10 -mmem
>>>> rtcwake: wakeup from "mem" using /dev/rtc0 at Tue Oct 29 11:54:30 2024
>>>> PM: suspend entry (s2idle)
>>>> Filesystems sync: 0.004 seconds
>>>> Freezing user space processes
>>>> Freezing user space processes completed (elapsed 0.007 seconds)
>>>> OOM killer disabled.
>>>> Freezing remaining freezable tasks
>>>> Freezing remaining freezable tasks completed (elapsed 0.004 seconds)
>>>>
>>>> ======================================================
>>>> WARNING: possible circular locking dependency detected
>>>> 6.12.0-rc4+ #9291 Not tainted
>>>> ------------------------------------------------------
>>>> rtcwake/1299 is trying to acquire lock:
>>>> ffff80008358a7f8 (rtnl_mutex){+.+.}-{3:3}, at: rtnl_lock+0x1c/0x28
>>>>
>>>> but task is already holding lock:
>>>> ffff000006136d68 (&q->q_usage_counter(io)#5){++++}-{0:0}, at:
>>>> virtblk_freeze+0x24/0x60
>>>>
>>>> which lock already depends on the new lock.
>>>>
>>>>
>>>> the existing dependency chain (in reverse order) is:
>>> This one looks a real thing, at least the added lockdep code works as
>>> expected, also the blk_mq_freeze_queue() use in virtio-blk's ->suspend()
>>> is questionable. I will take a further look.
>> Did you find a way to fix this one? I still observe such warnings in my
>> tests, even though your lockdep fixes are already merged to -next:
>> https://lore.kernel.org/all/20241031133723.303835-1-ming.lei@redhat.com/
> The lockdep fixes in ->next is just for making the added lockdep work
> correctly, and virtio-blk is another story.
>
> It might be fine to annotate it with blk_mq_freeze_queue_no_owner(),
> but it looks very fragile to call freeze queue in ->suspend(), and the lock
> is just kept as being grabbed in the whole suspend code path.
>
> Can you try the following patch?

Yes, this hides this lockdep warning, but imho it looks like a 
workaround, not a final fix.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>


> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 194417abc105..21488740eb15 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -1594,6 +1594,7 @@ static int virtblk_freeze(struct virtio_device *vdev)
>   
>   	/* Ensure no requests in virtqueues before deleting vqs. */
>   	blk_mq_freeze_queue(vblk->disk->queue);
> +	blk_mq_unfreeze_queue(vblk->disk->queue);
>   
>   	/* Ensure we don't receive any more interrupts */
>   	virtio_reset_device(vdev);
> @@ -1617,8 +1618,6 @@ static int virtblk_restore(struct virtio_device *vdev)
>   		return ret;
>   
>   	virtio_device_ready(vdev);
> -
> -	blk_mq_unfreeze_queue(vblk->disk->queue);
>   	return 0;
>   }
>   #endif
>
>
>
> Thanks,
> Ming
>
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland