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SubjectRe: irq lock inversion

* Tejun Heo <tj@kernel.org> wrote:

> > From: Jiri Kosina <jkosina@suse.cz>
> > Subject: lockdep: avoid false positives about irq-safety
> >
> > Commit 403a91b1 ("percpu: allow pcpu_alloc() to be called
> > with IRQs off") introduced this warning:
> >
> > =========================================================
> > [ INFO: possible irq lock inversion dependency detected ]
> > 2.6.32-rc5-tip-04815-g12f0f93-dirty #745
> > ---------------------------------------------------------
> > hub 1-3:1.0: state 7 ports 2 chg 0000 evt 0004
> > ksoftirqd/65/199 just changed the state of lock:
> > (pcpu_lock){..-...}, at: [<ffffffff81130e04>] free_percpu+0x38/0x104
> > but this lock took another, SOFTIRQ-unsafe lock in the past:
> > (vmap_area_lock){+.+...}
> >
> > and interrupts could create inverse lock ordering between them.
> >
> > This warning is bogus -- sched_init() is being called very early with IRQs
> > disabled, and the irqsave/restore code paths in pcpu_alloc() are only for early
> > init. The path can never be called from irq context once the early init
> > finishes. Rationale for this is explained in changelog of the commit mentioned
> > above.
> >
> > This problem can be encountered generally in any other early code running
> > with IRQs off and using irqsave/irqrestore.
> >
> > Reported-by: Yinghai Lu <yhlu.kernel@gmail.com>
> > Signed-off-by: Jiri Kosina <jkosina@suse.cz>
>
> Looks good to me. Ingo, what do you think?

Ugh, this explanation is _BOGUS_. As i said, taking a lock with irqs
disabled does _NOT_ mark a lock as 'irq safe' - if it did, we'd have
false positives left and right.

Read the lockdep message please, consider all the backtraces it prints,
it says something different.

Ingo


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