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

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Date:   Thu, 11 Oct 2018 17:50:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Eric Dumazet <edumazet@...gle.com>
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 08:24:49AM -0700, Eric Dumazet wrote:
> 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())

Right; it goes back a long long way... is:

  7c3576d261ce ("[PATCH] i386: Convert PDA into the percpu section")

early enough? That introduces percpu_from_op(), but arguably the
pda_from_op() it replaces was buggy already.