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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Nov 12, 2024 at 12:32:29PM +0100, Marek Szyprowski wrote:
> 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>

Thanks for the test!

It is actually not workaround, because what virtblk_freeze() needs is to drain
all in-flight IOs. One thing missed is to mark the queue as quiesced,
and I will post one formal patch with queue quiesce covered.


Thanks,
Ming