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 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?

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