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SubjectRe: Real deadlock being suppressed in sbitmap
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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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