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

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Message-ID: <6551c33f-b9e1-45ab-b420-d022d6e4e402@samsung.com>
Date: Tue, 12 Nov 2024 09:36:40 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Ming Lei <ming.lei@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org, Christoph
	Hellwig <hch@....de>, Peter Zijlstra <peterz@...radead.org>, Waiman Long
	<longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, Ingo Molnar
	<mingo@...hat.com>, Will Deacon <will@...nel.org>,
	linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH V2 3/3] block: model freeze & enter queue as lock for
 supporting lockdep

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@....de>
>>> Signed-off-by: Ming Lei <ming.lei@...hat.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/


Best regards

-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland