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@xxxxxxxxxx/ >>>>> >>>>> [3] queue_freeze & queue_enter deadlock in scsi >>>>> https://lore.kernel.org/linux-block/ZxG38G9BuFdBpBHZ@fedora/T/#u >>>>> >>>>> Reviewed-by: Christoph Hellwig <hch@xxxxxx> >>>>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> >>>> 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@xxxxxxxxxx/ > 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@xxxxxxxxxxx> Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> > 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
