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/ Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
