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SubjectRe: [PATCH 1/3] perf_event: fix race in perf_swevent_get_recursion_context()


Frederic Weisbecker wrote:
> On Mon, Jan 18, 2010 at 09:42:34PM +0800, Xiao Guangrong wrote:
>> It only disable preemption in perf_swevent_get_recursion_context()
>> it can't avoid race of hard-irq and NMI
>>
>> In this patch, we use atomic operation to avoid it and reduce
>> cpu_ctx->recursion size, it also make this patch no need diable
>> preemption
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
>
>
>
> I don't understand what is racy in what we have currently.
>

It's because hard-irq(we can handle interruption with interruption enabled)
and NMI are nested, for example:

int perf_swevent_get_recursion_context(void)
{
......
if (cpuctx->recursion[rctx]) {
put_cpu_var(perf_cpu_context);
return -1;
}
/*
* Another interruption handler/NMI will re-enter there if it
* happed, it make the recursion value chaotic
*/
cpuctx->recursion[rctx]++;
......
}
>
>
>> int perf_swevent_get_recursion_context(void)
>> {
>> - struct perf_cpu_context *cpuctx = &get_cpu_var(perf_cpu_context);
>> + struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context);
>> int rctx;
>>
>> if (in_nmi())
>> @@ -3933,13 +3933,8 @@ int perf_swevent_get_recursion_context(void)
>> else
>> rctx = 0;
>>
>> - if (cpuctx->recursion[rctx]) {
>> - put_cpu_var(perf_cpu_context);
>> + if (test_and_set_bit(rctx, &cpuctx->recursion))
>> return -1;
>
>
>
> This looks broken. We don't call back perf_swevent_put_recursion_context
> in fail case, so the bit won't ever be cleared once we recurse.
>

Um, i think we can't clear the bit in this fail case, consider below
sequence:

path A: path B

set bit but find the bit already set
atomic set bit |
| |
V |
handle SW event |
| V
V exit and not clear the bit
atomic clear bit
After A and B, the bit is still zero

Right? :-)

Thanks,
Xiao


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