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

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Message-ID: <20180711172848.GF72677@devbig577.frc2.facebook.com>
Date:   Wed, 11 Jul 2018 10:28:48 -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()

Hello, Sebastian.

On Wed, Jul 11, 2018 at 01:05:13PM +0200, Sebastian Andrzej Siewior wrote:
> > > 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?
> > 
> > No, this is correct. So on !RT kernels the spin_lock_irq() disables
> > interrupts and the raw_spin_lock() has the interrupts already disabled,
> > everything is good. On RT kernels the spin_lock_irq() does not disable
> > interrupts and the raw_spin_lock() acquires the lock with enabled
> > interrupts and lockdep complains properly.
> > lockdep sees the hardirq path via:

I feel weary about applying a patch which isn't needed in mainline,
especially without annotations or at least comments.  I suppose it may
not be too common but this can't be the only place which needs this
and using irqsave/restore spuriously in all those sites doesn't sound
like a good solution.  Is there any other way of handling this?

Thanks.

-- 
tejun