linux-kernel - Re: [PATCH 3/3] kcov: remove useless barrier()s

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Date:   Tue, 19 Sep 2017 14:57:22 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Victor Chibotaru <tchibo@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] kcov: remove useless barrier()s

On Tue, Sep 19, 2017 at 2:46 PM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
> As comment says barriers needed for preempt_schedule_irq() case
> where in_interrupt() returns false. But we don't use in_interrupt()
> since b274c0bb394c ("kcov: properly check if we are in an interrupt").
>
> Now we use in_task() which handles preempt_schedule_irq() case properly,
> thus no barrier required.


Are you sure in_task() handles preempt_schedule_irq() correctly?
They seem to differ only by SOFTIRQ_MASK vs SOFTIRQ_OFFSET, and that
only differs in local_bh_disable sections. But preempt_schedule_irq()
does not seem to have anything to do softirq/local_bh_disable. It's
called from real interrupts, right? So I would expect that in_task()
returns true in preempt_schedule_irq().


> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> ---
>  kernel/kcov.c | 10 ----------
>  1 file changed, 10 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 14cc8c1a7cad..b7fbcbef88c1 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -71,14 +71,6 @@ void notrace __sanitizer_cov_trace_pc(void)
>
>                 ip -= kaslr_offset();
>
> -               /*
> -                * There is some code that runs in interrupts but for which
> -                * in_interrupt() returns false (e.g. preempt_schedule_irq()).
> -                * READ_ONCE()/barrier() effectively provides load-acquire wrt
> -                * interrupts, there are paired barrier()/WRITE_ONCE() in
> -                * kcov_ioctl_locked().
> -                */
> -               barrier();
>                 area = t->kcov_area;
>                 /* The first word is number of subsequent PCs. */
>                 pos = READ_ONCE(area[0]) + 1;
> @@ -228,8 +220,6 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>                 /* Cache in task struct for performance. */
>                 t->kcov_size = kcov->size;
>                 t->kcov_area = kcov->area;
> -               /* See comment in __sanitizer_cov_trace_pc(). */
> -               barrier();
>                 WRITE_ONCE(t->kcov_mode, kcov->mode);
>                 t->kcov = kcov;
>                 kcov->t = t;
> --
> 2.13.5
>