Re: [PATCH v3 5/5] psi: introduce psi monitor

From: Suren Baghdasaryan <surenb@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Tejun Heo <tj@kernel.org>,
	lizefan@huawei.com, Johannes Weiner <hannes@cmpxchg.org>,
	axboe@kernel.dk, dennis@kernel.org,
	Dennis Zhou <dennisszhou@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jonathan Corbet <corbet@lwn.net>,
	cgroups@vger.kernel.org, linux-mm <linux-mm@kvack.org>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	kernel-team@android.com
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor
Date: Tue, 29 Jan 2019 10:25:58 -0800	[thread overview]
Message-ID: <CAJuCfpHyuY89Aw3YpQpaor2hKvdqJOH5sMoXNZ-yNAnAb+tY6A@mail.gmail.com> (raw)
In-Reply-To: <20190129151649.GA2997@hirez.programming.kicks-ass.net>

On Tue, Jan 29, 2019 at 7:16 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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.

Got it. Will change the comment to something like:

Order group->polling=0 before reading times[] in
collect_percpu_times() to detect possible race with hotpath that
modifies times[] before it sets group->polling=1 (see Race #1 in the
comments at the top).

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