Re: [tip:perf/core] perf: Fix race in perf_event_exec()

From: Peter Zijlstra <peterz@infradead.org>
To: Eric Dumazet <edumazet@google.com>
Cc: syzkaller <syzkaller@googlegroups.com>,
	LKML <linux-kernel@vger.kernel.org>,
	acme@redhat.com, Linus Torvalds <torvalds@linux-foundation.org>,
	Alexander Potapenko <glider@google.com>,
	Kostya Serebryany <kcc@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Sasha Levin <sasha.levin@oracle.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	jolsa@redhat.com, Ingo Molnar <mingo@kernel.org>,
	vincent.weaver@maine.edu, Thomas Gleixner <tglx@linutronix.de>,
	acme@kernel.org, Stephane Eranian <eranian@google.com>,
	linux-tip-commits@vger.kernel.org,
	Paul McKenney <paulmck@linux.vnet.ibm.com>
Subject: Re: [tip:perf/core] perf: Fix race in perf_event_exec()
Date: Thu, 7 Jan 2016 14:40:08 +0100	[thread overview]
Message-ID: <20160107134008.GZ6344@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <CANn89iLzUuR6wM=hOybM25d21PN=Fax5Ms7z2XeArTwAhZ6zqQ@mail.gmail.com>

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@zytor.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.