linux-kernel - Re: [PATCH v3 5/5] psi: introduce psi monitor

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Message-ID: <20190129151649.GA2997@hirez.programming.kicks-ass.net>
Date:   Tue, 29 Jan 2019 16:16:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     gregkh@...uxfoundation.org, tj@...nel.org, lizefan@...wei.com,
        hannes@...xchg.org, axboe@...nel.dk, dennis@...nel.org,
        dennisszhou@...il.com, mingo@...hat.com, akpm@...ux-foundation.org,
        corbet@....net, cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor

On Tue, Jan 29, 2019 at 01:38:43PM +0100, Peter Zijlstra wrote:
> On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote:
> > +			atomic_set(&group->polling, polling);
> > +			/*
> > +			 * Memory barrier is needed to order group->polling
> > +			 * write before times[] read in collect_percpu_times()
> > +			 */
> > +			smp_mb__after_atomic();
> 
> That's broken, smp_mb__{before,after}_atomic() can only be used on
> atomic RmW operations, something atomic_set() is _not_.

Also; the comment should explain _why_ not only what.