Messages in this thread | | | Subject | Re: Real deadlock being suppressed in sbitmap | From | Jens Axboe <> | Date | Mon, 14 Jan 2019 20:41:16 -0700 |
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On 1/14/19 8:23 PM, Ming Lei wrote: > Hi Steven, > > On Mon, Jan 14, 2019 at 12:14:14PM -0500, 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. > > Actually sbq->ws[i].wait is taken from a softirq context only in case > of single-queue, see __blk_mq_complete_request(). For multiple queue, > sbq->ws[i].wait is taken from hardirq context.
That's a good point, but that's just current implementation, we can't assume any of those relationsships. Any completion can happen from softirq or hardirq. So the patch is inadequate.
> Sounds the correct fix may be the following one, and the irqsave cost > should be fine given sbitmap_deferred_clear is only triggered when one > word is run out of.
Yes, the _bh() variant isn't going to cut it. Can you send this patch against Linus's master?
-- Jens Axboe
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