From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <edumazet@google.com>
Cc: Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 2/2] x86/percpu: Fix this_cpu_read()
Date: Thu, 11 Oct 2018 17:50:34 +0200 [thread overview]
Message-ID: <20181011155034.GC9867@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CANn89iJsD=rKTjiGF_EEoSh9WmiifOjxu2mcqSv8Sx-NT7zixQ@mail.gmail.com>
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@google.com> wrote:
> >
> > On Thu, Oct 11, 2018 at 3:45 AM Peter Zijlstra <peterz@infradead.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@google.com>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> >
> >
> > Acked-by: Eric Dumazet <edumazet@google.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.
next prev parent reply other threads:[~2018-10-11 15:50 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-11 10:38 [PATCH 0/2] x86/tsc: Fix native_sched_clock() Peter Zijlstra
2018-10-11 10:38 ` [PATCH 1/2] x86/tsc: Force inlining of cyc2ns bits Peter Zijlstra
2018-10-14 9:15 ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
2018-10-11 10:38 ` [PATCH 2/2] x86/percpu: Fix this_cpu_read() Peter Zijlstra
2018-10-11 15:02 ` Eric Dumazet
2018-10-11 15:24 ` Eric Dumazet
2018-10-11 15:50 ` Peter Zijlstra [this message]
2018-10-11 16:08 ` Eric Dumazet
2018-10-14 9:16 ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
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