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

Dmitry Vyukov
2017-09-19T13:54:29Z

On Tue, Sep 19, 2017 at 3:52 PM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
>
>
> On 09/19/2017 03:57 PM, Dmitry Vyukov wrote:
>> On Tue, Sep 19, 2017 at 2:46 PM, Andrey Ryabinin
>> <aryabinin@virtuozzo.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@virtuozzo.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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