Messages in this thread | | | Subject | Re: Real deadlock being suppressed in sbitmap | From | Jens Axboe <> | Date | Mon, 14 Jan 2019 12:43:56 -0700 |
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On 1/14/19 10:14 AM, Steven Rostedt wrote: > It was brought to my attention (by this creating a splat in the RT tree > too) this code: > > static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index) > { > unsigned long mask, val; > unsigned long __maybe_unused flags; > bool ret = false; > > /* Silence bogus lockdep warning */ > #if defined(CONFIG_LOCKDEP) > local_irq_save(flags); > #endif > spin_lock(&sb->map[index].swap_lock); > > Commit 58ab5e32e6f ("sbitmap: silence bogus lockdep IRQ warning") > states the following: > > For this case, it's a false positive. The swap_lock is used from process > context only, when we swap the bits in the word and cleared mask. We > also end up doing that when we are getting a driver tag, from the > blk_mq_mark_tag_wait(), and from there we hold the waitqueue lock with > IRQs disabled. However, this isn't from an actual IRQ, it's still > process context. > > The thing is, lockdep doesn't define a lock as "irq-safe" based on it > being taken under interrupts disabled or not. It detects when locks are > used in actual interrupts. Further in that commit we have this: > > [ 106.097386] fio/1043 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire: > [ 106.098231] 000000004c43fa71 > (&(&sb->map[i].swap_lock)->rlock){+.+.}, at: sbitmap_get+0xd5/0x22c > [ 106.099431] > [ 106.099431] and this task is already holding: > [ 106.100229] 000000007eec8b2f > (&(&hctx->dispatch_wait_lock)->rlock){....}, at: > blk_mq_dispatch_rq_list+0x4c1/0xd7c > [ 106.101630] which would create a new lock dependency: > [ 106.102326] (&(&hctx->dispatch_wait_lock)->rlock){....} -> > (&(&sb->map[i].swap_lock)->rlock){+.+.} > > Saying that you are trying to take the swap_lock while holding the > dispatch_wait_lock. > > > [ 106.103553] but this new dependency connects a SOFTIRQ-irq-safe lock: > [ 106.104580] (&sbq->ws[i].wait){..-.} > > Which means that there's already a chain of: > > sbq->ws[i].wait -> dispatch_wait_lock > > [ 106.104582] > [ 106.104582] ... which became SOFTIRQ-irq-safe at: > [ 106.105751] _raw_spin_lock_irqsave+0x4b/0x82 > [ 106.106284] __wake_up_common_lock+0x119/0x1b9 > [ 106.106825] sbitmap_queue_wake_up+0x33f/0x383 > [ 106.107456] sbitmap_queue_clear+0x4c/0x9a > [ 106.108046] __blk_mq_free_request+0x188/0x1d3 > [ 106.108581] blk_mq_free_request+0x23b/0x26b > [ 106.109102] scsi_end_request+0x345/0x5d7 > [ 106.109587] scsi_io_completion+0x4b5/0x8f0 > [ 106.110099] scsi_finish_command+0x412/0x456 > [ 106.110615] scsi_softirq_done+0x23f/0x29b > [ 106.111115] blk_done_softirq+0x2a7/0x2e6 > [ 106.111608] __do_softirq+0x360/0x6ad > [ 106.112062] run_ksoftirqd+0x2f/0x5b > [ 106.112499] smpboot_thread_fn+0x3a5/0x3db > [ 106.113000] kthread+0x1d4/0x1e4 > [ 106.113457] ret_from_fork+0x3a/0x50 > > > We see that sbq->ws[i].wait was taken from a softirq context. > > > > [ 106.131226] Chain exists of: > [ 106.131226] &sbq->ws[i].wait --> > &(&hctx->dispatch_wait_lock)->rlock --> > &(&sb->map[i].swap_lock)->rlock > > This is telling us that we now have a chain of: > > sbq->ws[i].wait -> dispatch_wait_lock -> swap_lock > > [ 106.131226] > [ 106.132865] Possible interrupt unsafe locking scenario: > [ 106.132865] > [ 106.133659] CPU0 CPU1 > [ 106.134194] ---- ---- > [ 106.134733] lock(&(&sb->map[i].swap_lock)->rlock); > [ 106.135318] local_irq_disable(); > [ 106.136014] lock(&sbq->ws[i].wait); > [ 106.136747] > lock(&(&hctx->dispatch_wait_lock)->rlock); > [ 106.137742] <Interrupt> > [ 106.138110] lock(&sbq->ws[i].wait); > [ 106.138625] > [ 106.138625] *** DEADLOCK *** > [ 106.138625] > > I need to make this more than just two levels deep. Here's the issue: > > > CPU0 CPU1 CPU2 > ---- ---- ---- > lock(swap_lock) > local_irq_disable() > lock(dispatch_lock); > local_irq_disable() > lock(sbq->ws[i].wait) > lock(dispatch_lock) > lock(swap_lock) > <interrupt> > lock(sbq->ws[i].wait) > > > DEADLOCK! > > In other words, it is not bogus, and can be a real potential for a > deadlock. Please talk with the lockdep maintainers before saying > there's a bogus deadlock, because lockdep is seldom wrong.
Thanks Steven, your analysis looks good. I got fooled by the fact that the path where we do grab them both is never in irq/soft-irq context, but that doesn't change the fact that the wq lock IS grabbed in irq context.
Patch also looks good, but I see Linus already applied it.
-- Jens Axboe
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