From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
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
Subject: Re: [tip:perf/core] perf: Fix race in perf_event_exec()
Date: Thu, 7 Jan 2016 08:46:25 -0800 [thread overview]
Message-ID: <20160107164625.GQ3818@linux.vnet.ibm.com> (raw)
In-Reply-To: <CANn89iKb7LJ2HgQEJeXZd05vc3QXqgAp9bVb0j3r2N2QzJAZcA@mail.gmail.com>
On Thu, Jan 07, 2016 at 11:36:21AM -0500, Eric Dumazet wrote:
> On Thu, Jan 7, 2016 at 11:26 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Thu, Jan 07, 2016 at 02:40:08PM +0100, Peter Zijlstra wrote:
> >> 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.
> >
> > What Peter said!
> >
> > The current implementation would let you get away with IRQs disabled
> > (aside from lockdep splats), but it is easy to imagine implementations
> > that do not, hence the restriction.
> >
> > Is the extra rcu_read_lock() and rcu_read_unlock() causing a performance
> > problem? If so, one option would be use of synchronize_sched() and
> > call_rcu_sched(), which explicitly recognize disabled IRQs as read-side
> > critical sections. Of course, this might well put you in a position
> > where you would like to use IRQ disabling in some cases, BH disabling
> > (or softirq context) in others, and rcu_read_lock() in yet other cases.
>
> I guess I could not see why the rcu_read_lock() was needed around this loop.
>
> rcu_read_lock();
> for ((ctxn) = 0; (ctxn) < perf_nr_task_contexts; (ctxn)++)
> perf_event_enable_on_exec(ctxn);
> rcu_read_unlock();
>
> But apparently it is needed in perf_event_enable_on_exec() and we avoid calling
> rcu_read_lock() and rcu_read_unlock() (perf_nr_task_contexts - 1 ) times
If the overhead of rcu_read_lock() and rcu_read_unlock() really are
a noticeable fraction of the loop overhead, and if perf_nr_task_contexts
is large, then you might well have a problem. I still suggest actually
measuring it, but you know me. ;-)
> > If so, the good news is that RCU actually now supports this efficiently.
> > There is the shiny new synchronize_rcu_mult() macro that can be used
> > as follows to wait for all three types of grace periods:
> >
> > synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
> >
> > This would wait concurrently for all three grace periods to elapse.
>
> Interesting ;)
;-) ;-) ;-)
Thanx, Paul
next prev parent reply other threads:[~2016-01-07 16:46 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-04 20:04 use-after-free in __perf_install_in_context Dmitry Vyukov
2015-12-04 20:32 ` Alexei Starovoitov
2015-12-04 21:00 ` Dmitry Vyukov
2015-12-07 11:04 ` Dmitry Vyukov
2015-12-07 11:06 ` Dmitry Vyukov
2015-12-07 11:24 ` Dmitry Vyukov
2015-12-07 15:36 ` Peter Zijlstra
2015-12-07 16:09 ` Dmitry Vyukov
2015-12-08 3:24 ` Alexei Starovoitov
2015-12-08 16:12 ` Dmitry Vyukov
2015-12-08 17:54 ` Alexei Starovoitov
2015-12-08 17:56 ` Dmitry Vyukov
2015-12-08 18:05 ` Alexei Starovoitov
2015-12-08 18:35 ` Dmitry Vyukov
2015-12-08 19:56 ` Alexei Starovoitov
2015-12-09 9:17 ` Dmitry Vyukov
2015-12-10 3:54 ` Alexei Starovoitov
2015-12-10 9:02 ` Peter Zijlstra
2015-12-10 17:03 ` Alexei Starovoitov
2015-12-11 8:14 ` Ingo Molnar
2015-12-15 13:11 ` Dmitry Vyukov
2015-12-08 16:44 ` Peter Zijlstra
2015-12-08 19:14 ` Dmitry Vyukov
2015-12-10 19:57 ` Peter Zijlstra
2015-12-15 13:09 ` Dmitry Vyukov
2015-12-17 14:06 ` Peter Zijlstra
2015-12-17 14:08 ` Dmitry Vyukov
2015-12-17 14:26 ` Peter Zijlstra
2015-12-17 14:28 ` Peter Zijlstra
2015-12-17 14:35 ` Dmitry Vyukov
2015-12-17 14:43 ` Peter Zijlstra
2015-12-31 17:15 ` Dmitry Vyukov
2016-01-05 12:17 ` Peter Zijlstra
2016-01-08 8:40 ` Dmitry Vyukov
2016-01-08 10:28 ` Dmitry Vyukov
2016-01-06 18:46 ` [tip:perf/core] perf: Fix race in perf_event_exec() tip-bot for Peter Zijlstra
2016-01-06 18:56 ` Eric Dumazet
2016-01-07 13:40 ` Peter Zijlstra
2016-01-07 16:26 ` Paul E. McKenney
2016-01-07 16:36 ` Eric Dumazet
2016-01-07 16:46 ` Paul E. McKenney [this message]
2015-12-08 16:22 ` use-after-free in __perf_install_in_context Peter Zijlstra
2015-12-08 18:57 ` Ingo Molnar
2015-12-09 9:05 ` Peter Zijlstra
2015-12-08 16:27 ` Peter Zijlstra
2015-12-08 16:50 ` Dmitry Vyukov
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