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

[Xiao Guangrong]

Frederic Weisbecker wrote:

> 
> I still don't understand the problem.
> 
> It's not like a fight between different cpus, it's a local per cpu
> fight.
> 
> NMIs can't nest other NMIs but hardirq can nest another hardirqs,
> we don't care much about these though.
> So let's imagine the following sequence, a fight between nested
> hardirqs:
> 
> cpuctx->recursion[irq] initially = 0
> 
> Interrupt (level 0):
> 
>        if (cpuctx->recursion[rctx]) {
>                put_cpu_var(perf_cpu_context);
>                return -1;
>        }
> 
> Interrupt (level 1):
> 
> 
> 	cpuctx->recursion[rctx]++; // = 1
> 
> 	...
> 	do something
> 	...
> 	cpuctx->recursion[rctx]--; // = 0
> 
> End Interrupt (level 1)
> 
> 	cpuctx->recursion[rctx]++; // = 1
> 
>         ...
>         do something
>         ...
>         cpuctx->recursion[rctx]--; // = 0
> 
> End interrupt (level 0)
> 
> Another sequence could be Interrupt level 0 has
> already incremented recursion and we are interrupted by
> irq level 1 which then won't be able to get the recursion
> context. But that's not a big deal I think.
> 

Thanks Frederic, i forget this feature of nesting of hard-irq :-(

Sorry for disturb you all

- Xiao
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