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

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Date:   Tue, 19 Sep 2017 15:54:06 +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 3:52 PM, Andrey Ryabinin
<aryabinin@...tuozzo.com> wrote:
>
>
> On 09/19/2017 03:57 PM, Dmitry Vyukov wrote:
>> 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().
>
> Indeed, you're right. I checked this only on !PREEMPT kernel, where this worked.
>
> Still, I think that barrier() in __sanitizer_cov_trace_pc() is not needed. AFAIU it needed
> to make sure that load of t->kcov_area isn't moved before load of t->kcov_mode, but I don't
> think that compiler is allowed to make such reorder. That would be a bug in the compiler.


Why? C compiler is allowed to fuse/reorder loads from the same base
object. Also stores can be reordered.


>>> 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
>>>
>
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