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: >> >> 1. On Samsung Exynos 4412-based Odroid U3 board (ARM 32bit), observed >> when booting it: >> >> ====================================================== >> WARNING: possible circular locking dependency detected >> 6.12.0-rc4-00037-gf1be1788a32e #9290 Not tainted >> ------------------------------------------------------ >> find/1284 is trying to acquire lock: >> cf3b8534 (&mm->mmap_lock){++++}-{3:3}, at: __might_fault+0x30/0x70 >> >> but task is already holding lock: >> c203a0c8 (&sb->s_type->i_mutex_key#2){++++}-{3:3}, at: >> iterate_dir+0x30/0x140 >> >> which lock already depends on the new lock. >> >> >> the existing dependency chain (in reverse order) is: >> >> -> #4 (&sb->s_type->i_mutex_key#2){++++}-{3:3}: >> down_write+0x44/0xc4 >> start_creating+0x8c/0x170 >> debugfs_create_dir+0x1c/0x178 >> blk_register_queue+0xa0/0x1c0 >> add_disk_fwnode+0x210/0x434 >> brd_alloc+0x1cc/0x210 >> brd_init+0xac/0x104 >> do_one_initcall+0x64/0x30c >> kernel_init_freeable+0x1c4/0x228 >> kernel_init+0x1c/0x12c >> ret_from_fork+0x14/0x28 >> >> -> #3 (&q->debugfs_mutex){+.+.}-{3:3}: >> __mutex_lock+0x94/0x94c >> mutex_lock_nested+0x1c/0x24 >> blk_mq_init_sched+0x140/0x204 >> elevator_init_mq+0xb8/0x130 >> add_disk_fwnode+0x3c/0x434 > The above chain can be cut by the following patch because disk state > can be thought as DEAD in add_disk(), can you test it? Seems to be fixing this issue. Feel free to add: Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> > diff --git a/block/elevator.c b/block/elevator.c > index 4122026b11f1..efa6ff941a25 100644 > --- a/block/elevator.c > +++ b/block/elevator.c > @@ -600,12 +600,14 @@ void elevator_init_mq(struct request_queue *q) > * requests, then no need to quiesce queue which may add long boot > * latency, especially when lots of disks are involved. > */ > - blk_mq_freeze_queue(q); > + if (__blk_freeze_queue_start(q)) > + blk_freeze_acquire_lock(q, true, false); > blk_mq_cancel_work_sync(q); > > err = blk_mq_init_sched(q, e); > > - blk_mq_unfreeze_queue(q); > + if (__blk_mq_unfreeze_queue(q, false)) > + blk_unfreeze_release_lock(q, true, false); > > if (err) { > pr_warn("\"%s\" elevator initialization failed, " > > ... > >> 2 locks held by find/1284: >> #0: c3df1e88 (&f->f_pos_lock){+.+.}-{3:3}, at: fdget_pos+0x88/0xd0 >> #1: c203a0c8 (&sb->s_type->i_mutex_key#2){++++}-{3:3}, at: >> iterate_dir+0x30/0x140 >> >> stack backtrace: >> CPU: 1 UID: 0 PID: 1284 Comm: find Not tainted >> 6.12.0-rc4-00037-gf1be1788a32e #9290 >> Hardware name: Samsung Exynos (Flattened Device Tree) >> Call trace: >> unwind_backtrace from show_stack+0x10/0x14 >> show_stack from dump_stack_lvl+0x68/0x88 >> dump_stack_lvl from print_circular_bug+0x31c/0x394 >> print_circular_bug from check_noncircular+0x16c/0x184 >> check_noncircular from __lock_acquire+0x158c/0x2970 >> __lock_acquire from lock_acquire+0x130/0x384 >> lock_acquire from __might_fault+0x50/0x70 >> __might_fault from filldir64+0x94/0x28c >> filldir64 from dcache_readdir+0x174/0x260 >> dcache_readdir from iterate_dir+0x64/0x140 >> iterate_dir from sys_getdents64+0x60/0x130 >> sys_getdents64 from ret_fast_syscall+0x0/0x1c >> Exception stack(0xf22b5fa8 to 0xf22b5ff0) >> 5fa0: 004b4fa0 004b4f80 00000004 004b4fa0 00008000 >> 00000000 >> 5fc0: 004b4fa0 004b4f80 00000001 000000d9 00000000 004b4af0 00000000 >> 000010ea >> 5fe0: 004b1eb4 bea05af0 b6da4b08 b6da4a28 >> >> --->8--- >> >> >> 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. > > > Thanks, > Ming > > Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
