linux-kernel - Re: [tip:perf/core] perf: Fix race in perf_event

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Date:	Wed, 6 Jan 2016 13:56:56 -0500
From:	Eric Dumazet <edumazet@...gle.com>
To:	syzkaller <syzkaller@...glegroups.com>,
	Eric Dumazet <edumazet@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, acme@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>, jolsa@...hat.com,
	Ingo Molnar <mingo@...nel.org>, vincent.weaver@...ne.edu,
	Thomas Gleixner <tglx@...utronix.de>, acme@...nel.org,
	Stephane Eranian <eranian@...gle.com>
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Fix race in perf_event_exec()

On Wed, Jan 6, 2016 at 1:46 PM, tip-bot for Peter Zijlstra
<tipbot@...or.com> wrote:

>
> This is because context switches can swap the task_struct::perf_event_ctxp[]
> pointer around. Therefore you have to either disable preemption when looking
> at current, or hold ctx->lock.
>


>
>  void perf_event_exec(void)
>  {
> -       struct perf_event_context *ctx;
>         int ctxn;
>
>         rcu_read_lock();

Do we still need this rcu_read_lock(), if perf_event_enable_on_exec()
uses  local_irq_save( ?

> -       for_each_task_context_nr(ctxn) {
> -               ctx = current->perf_event_ctxp[ctxn];
> -               if (!ctx)
> -                       continue;
> -
> -               perf_event_enable_on_exec(ctx);
> -       }
> +       for_each_task_context_nr(ctxn)
> +               perf_event_enable_on_exec(ctxn);
>         rcu_read_unlock();
>  }
>
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