linux-kernel - Re: [PATCH 1/3] perf_event: fix race in perf_swevent_get_recursion

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Date:	Mon, 18 Jan 2010 17:41:30 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] perf_event: fix race in
	perf_swevent_get_recursion_context()

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@...fujitsu.com>



I don't understand what is racy in what we have currently.



>  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.

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