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? 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
