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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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