linux-kernel - Re: [PATCH 2/2] x86/percpu: Fix this_cpu

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Date:   Thu, 11 Oct 2018 08:24:49 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 2/2] x86/percpu: Fix this_cpu_read()

On Thu, Oct 11, 2018 at 8:02 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Oct 11, 2018 at 3:45 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Eric reported that a sequence count loop using this_cpu_read() got
> > optimized out. This is wrong, this_cpu_read() must imply READ_ONCE()
> > because the interface is IRQ-safe, therefore an interrupt can have
> > changed the per-cpu value.
> >
> > Fixes: 59eaef78bfea ("x86/tsc: Remodel cyc2ns to use seqcount_latch()")
> > Reported-by: Eric Dumazet <edumazet@...gle.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>
>
> Acked-by: Eric Dumazet <edumazet@...gle.com>

Actually the Fixes: tag seems funky.

Bug was not added by 59eaef78bfea

Your patch probably needs to be backported to older versions of linux,
just to be safe, since
we might have other places where authors relied on this_cpu_read()
semantic (different than this_cpu_read_stable())