Messages in this thread | | | Subject | Re: lockdep false positive? -- firewire-core transaction timer vs. scsi-core host lock | From | Peter Zijlstra <> | Date | Mon, 16 Aug 2010 23:42:19 +0200 |
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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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