linux-kernel - Re: [PATCH] cgroup: use irqsave in cgroup_rstat_flush

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Message-ID: <20180703202424.GB72677@devbig577.frc2.facebook.com>
Date:   Tue, 3 Jul 2018 13:24:24 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] cgroup: use irqsave in cgroup_rstat_flush_locked()

(cc'ing Peter and Ingo for lockdep)

Hello, Sebastian.

On Tue, Jul 03, 2018 at 06:45:44PM +0200, Sebastian Andrzej Siewior wrote:
> All callers of cgroup_rstat_flush_locked() acquire cgroup_rstat_lock
> either with spin_lock_irq() or spin_lock_irqsave().

So, irq is always disabled in cgroup_rstat_flush_locked().

> cgroup_rstat_flush_locked() itself acquires cgroup_rstat_cpu_lock which
> is a raw_spin_lock. This lock is also acquired in cgroup_rstat_updated()
> in IRQ context and therefore requires _irqsave() locking suffix in
> cgroup_rstat_flush_locked().

Yes, the cpu locks should be irqsafe too; however, as irq is always
disabled in that function, save/restore is redundant, no?

> Since there is no difference between spin_lock_t and raw_spin_lock_t
> on !RT lockdep does not complain here. On RT lockdep complains because
> the interrupts were not disabled here and a deadlock is possible.

We at least used to do this in the kernel - manipulating irqsafe locks
with spin_lock/unlock() if the irq state is known, whether enabled or
disabled, and ISTR lockdep being smart enough to track actual irq
state to determine irq safety.  Am I misremembering or is this
different on RT kernels?

Thanks.

-- 
tejun