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

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Date:	Thu, 7 Jan 2016 14:40:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	syzkaller <syzkaller@...glegroups.com>,
	LKML <linux-kernel@...r.kernel.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>,
	linux-tip-commits@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [tip:perf/core] perf: Fix race in perf_event_exec()

On Wed, Jan 06, 2016 at 01:56:56PM -0500, Eric Dumazet wrote:
> 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( ?

Strictly speaking we should not rely on the fact that RCU grace periods
do not progress with IRQs disabled, so yes.
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