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

[Andrey Ryabinin]

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


> 
>> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
>> ---
>>  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
>>