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SubjectRe: lockdep false positive? -- firewire-core transaction timer vs. scsi-core host lock
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On Mon, 2010-08-16 at 20:42 +0200, Stefan Richter wrote:
> What does this all mean? How to act on it?
>
> =========================================================
> [ INFO: possible irq lock inversion dependency detected ]
> 2.6.35 #1
> ---------------------------------------------------------
> swapper/0 just changed the state of lock:
> (&t->split_timeout_timer){+.-...}, at: [<c1032228>] run_timer_softirq+0x112/0x21c
> but this lock was taken by another, HARDIRQ-safe lock in the past:
> (&(shost->host_lock)->rlock){-.-...}
>
> and interrupts could create inverse lock ordering between them.


It says the locks nest like:

(&(shost->host_lock)->rlock){-.-...}
(&t->split_timeout_timer){+.-...}

But:


> -> (&(shost->host_lock)->rlock){-.-...} ops: 123308 {
> IN-HARDIRQ-W at:
> [<c104a080>] __lock_acquire+0x612/0x153e
> [<c104b008>] lock_acquire+0x5c/0x73
> [<c1268b57>] _raw_spin_lock_irqsave+0x2e/0x3e
> [<c11ad508>] scsi_eh_scmd_add+0x25/0x8a

...

vs

> -> (&t->split_timeout_timer){+.-...} ops: 14281 {
> HARDIRQ-ON-W at:
> [<c104a100>] __lock_acquire+0x692/0x153e
> [<c104b008>] lock_acquire+0x5c/0x73
> [<c1032291>] run_timer_softirq+0x17b/0x21c
> [<c102defb>] __do_softirq+0x8b/0x10a
> [<c102dfa5>] do_softirq+0x2b/0x43

...


So shost->host_lock->rlock is a lock used in hardirq context but
split_timeout_timer is a !irq-safe lock.

Which means that it now worries the following can happen:

softirq:
spin_lock(&t->split_timeout_timer);

IRQ:
spin_lock(&(shost->host_lock)->rlock);
spin_lock(&t->split_timeout_timer);

Now, the thing is that split_timeout_timer is a fake lock used to
annotate timers, its use is to connect lock chains from within the timer
callback to del_timer_sync() callers, to detect deadlocks.

Now, I can't seem to remember why del_timer_sync() explicitly disables
IRQs but call_timer_fn() does not, Johill, happen to remember?


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