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SubjectReal deadlock being suppressed in sbitmap
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.

-- Steve

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