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

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Date:   Thu, 11 Oct 2018 08:02:48 -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 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>

> ---
>  arch/x86/include/asm/percpu.h |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> --- a/arch/x86/include/asm/percpu.h
> +++ b/arch/x86/include/asm/percpu.h
> @@ -185,22 +185,22 @@ do {                                                                      \
>         typeof(var) pfo_ret__;                          \
>         switch (sizeof(var)) {                          \
>         case 1:                                         \
> -               asm(op "b "__percpu_arg(1)",%0"         \
> +               asm volatile(op "b "__percpu_arg(1)",%0"\
>                     : "=q" (pfo_ret__)                  \
>                     : "m" (var));                       \
>                 break;                                  \
>         case 2:                                         \
> -               asm(op "w "__percpu_arg(1)",%0"         \
> +               asm volatile(op "w "__percpu_arg(1)",%0"\
>                     : "=r" (pfo_ret__)                  \
>                     : "m" (var));                       \
>                 break;                                  \
>         case 4:                                         \
> -               asm(op "l "__percpu_arg(1)",%0"         \
> +               asm volatile(op "l "__percpu_arg(1)",%0"\
>                     : "=r" (pfo_ret__)                  \
>                     : "m" (var));                       \
>                 break;                                  \
>         case 8:                                         \
> -               asm(op "q "__percpu_arg(1)",%0"         \
> +               asm volatile(op "q "__percpu_arg(1)",%0"\
>                     : "=r" (pfo_ret__)                  \
>                     : "m" (var));                       \
>                 break;                                  \
>
>