linux-kernel - Re: [PATCH 2/4] irq_work: Fix irq_work

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Date:   Tue, 12 Nov 2019 00:17:06 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/4] irq_work: Fix irq_work_claim() ordering

On Mon, Nov 11, 2019 at 08:20:05AM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <frederic@...nel.org> wrote:
> 
> > When irq_work_claim() finds IRQ_WORK_PENDING flag already set, we just
> > return and don't raise a new IPI. We expect the destination to see
> > and handle our latest updades thanks to the pairing atomic_xchg()
> > in irq_work_run_list().
> > 
> > But cmpxchg() doesn't guarantee a full memory barrier upon failure. So
> > it's possible that the destination misses our latest updates.
> > 
> > So use atomic_fetch_or() instead that is unconditionally fully ordered
> > and also performs exactly what we want here and simplify the code.
> 
> Just curious, how was this bug found - in the wild, or via code review?

Well, I wanted to make sure the nohz kcpustat patches are safe and I had
a last minute doubt about that irq work scenario. So I would say code
review :)