From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
linux-kernel <linux-kernel@vger.kernel.org>,
"Joel Fernandes, Google" <joel@joelfernandes.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
paulmck <paulmck@linux.ibm.com>,
Boqun Feng <boqun.feng@gmail.com>,
Will Deacon <will.deacon@arm.com>,
David Howells <dhowells@redhat.com>,
Alan Stern <stern@rowland.harvard.edu>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
Date: Fri, 16 Aug 2019 13:41:59 -0400 (EDT) [thread overview]
Message-ID: <241506096.21688.1565977319832.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20190816130440.07cc0a30@oasis.local.home>
----- On Aug 16, 2019, at 1:04 PM, rostedt rostedt@goodmis.org wrote:
> On Fri, 16 Aug 2019 17:48:59 +0100
> Valentin Schneider <valentin.schneider@arm.com> wrote:
>
>> On 16/08/2019 17:25, Steven Rostedt wrote:
>> >> Also, write and read to/from those variables should be done with
>> >> WRITE_ONCE() and READ_ONCE(), given that those are read within tracing
>> >> probes without holding the sched_register_mutex.
>> >>
>> >
>> > I understand the READ_ONCE() but is the WRITE_ONCE() truly necessary?
>> > It's done while holding the mutex. It's not that critical of a path,
>> > and makes the code look ugly.
>> >
>>
>> I seem to recall something like locking primitives don't protect you from
>> store tearing / invented stores, so if you can have concurrent readers
>> using READ_ONCE(), there should be a WRITE_ONCE() on the writer side, even
>> if it's done in a critical section.
>
> But for this, it really doesn't matter. The READ_ONCE() is for going
> from 0->1 or 1->0 any other change is the same as 1.
Let's consider this "invented store" scenario (even though as I noted in my
earlier email, I suspect this particular instance of the code does not appear
to fit the requirements to generate this in the wild with current compilers):
intial state:
sched_tgid_ref = 10;
Thread A Thread B
registration tracepoint probe
sched_tgid_ref++
- compiler decides to invent a
store: sched_tgid_ref = 0
READ_ONCE(sched_tgid_ref) -> observes 0,
but should really see either 10 or 11.
- compiler stores 11 into
sched_tgid_ref
This kind of scenario could cause spurious missing data in the middle of a
trace caused by another user trying to increment the reference count, which
is really unexpected.
A similar scenario can happen for "store tearing" if the compiler decides
to break the store into many stores close to the 16-bit overflow value when
updating a 32-bit reference count. Spurious 1 -> 0 -> 1 transitions could be
observed by readers.
> When we enable trace events, we start recording the tasks comms such
> that we can possibly map them to the pids. When we disable trace
> events, we stop recording the comms so that we don't overwrite the
> cache when not needed. Note, if more than the max cache of tasks are
> recorded during a session, we are likely to miss comms anyway.
>
> Thinking about this more, the READ_ONCE() and WRTIE_ONCE() are not even
> needed, because this is just a best effort anyway.
If you choose not to use READ_ONCE(), then the "load tearing" issue can
cause similar spurious 1 -> 0 -> 1 transitions near 16-bit counter
overflow as described above. The "Invented load" also becomes an issue,
because the compiler could use the loaded value for a branch, and re-load
that value between two branches which are expected to use the same value,
effectively generating a corrupted state.
I think we need a statement about whether READ_ONCE/WRITE_ONCE should
be used in this kind of situation, or if we are fine dealing with the
awkward compiler side-effects when they will occur.
Thanks,
Mathieu
>
> The only real fix was to move the check into the mutex protect area,
> because that can cause a real bug if there was a race.
>
> {
> - bool sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
> + bool sched_register;
> +
> mutex_lock(&sched_register_mutex);
> + sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
>
> Thus, I'd like to see a v2 of this patch without the READ_ONCE() or
> WRITE_ONCE() added.
>
> -- Steve
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2019-08-16 17:42 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-18 10:29 WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16 0:11 ` syzbot
2019-08-16 14:26 ` [PATCH 1/1] Fix: trace sched switch start/stop racy updates Mathieu Desnoyers
2019-08-16 16:25 ` Steven Rostedt
2019-08-16 16:48 ` Valentin Schneider
2019-08-16 17:04 ` Steven Rostedt
2019-08-16 17:41 ` Mathieu Desnoyers [this message]
2019-08-16 19:18 ` Steven Rostedt
2019-08-16 19:19 ` Alan Stern
2019-08-16 20:44 ` Joel Fernandes
2019-08-16 20:49 ` Thomas Gleixner
2019-08-16 20:57 ` Joel Fernandes
2019-08-16 22:27 ` Valentin Schneider
2019-08-16 22:57 ` Linus Torvalds
2019-08-17 1:41 ` Mathieu Desnoyers
2019-08-17 4:52 ` Paul E. McKenney
2019-08-17 8:28 ` Linus Torvalds
2019-08-17 8:44 ` Linus Torvalds
2019-08-17 15:02 ` Mathieu Desnoyers
2019-08-17 20:03 ` Valentin Schneider
2019-08-17 23:00 ` Paul E. McKenney
2019-08-19 10:34 ` Valentin Schneider
2019-08-17 22:28 ` Paul E. McKenney
2019-08-20 14:01 ` Peter Zijlstra
2019-08-20 20:31 ` Paul E. McKenney
2019-08-20 20:39 ` Peter Zijlstra
2019-08-20 20:52 ` Paul E. McKenney
2019-08-16 21:04 ` Linus Torvalds
2019-08-17 1:36 ` Mathieu Desnoyers
2019-08-17 2:13 ` Steven Rostedt
2019-08-17 14:40 ` Mathieu Desnoyers
2019-08-17 15:26 ` Steven Rostedt
2019-08-17 15:55 ` Mathieu Desnoyers
2019-08-17 16:40 ` Steven Rostedt
2019-08-17 22:06 ` Paul E. McKenney
2019-08-17 8:08 ` Linus Torvalds
2019-08-20 13:56 ` Peter Zijlstra
2019-08-20 20:29 ` Paul E. McKenney
2019-08-21 10:32 ` Will Deacon
2019-08-21 13:23 ` Paul E. McKenney
2019-08-21 13:32 ` Will Deacon
2019-08-21 13:56 ` Paul E. McKenney
2019-08-21 16:22 ` Will Deacon
2019-08-21 15:33 ` Peter Zijlstra
2019-08-21 15:48 ` Mathieu Desnoyers
2019-08-21 16:14 ` Paul E. McKenney
2019-08-21 19:03 ` Joel Fernandes
2019-09-09 6:21 ` Herbert Xu
2019-08-16 20:49 ` Steven Rostedt
2019-08-16 20:59 ` Joel Fernandes
2019-08-17 1:25 ` Mathieu Desnoyers
2019-08-18 9:15 ` stable markup was " Pavel Machek
2019-08-16 17:19 ` Mathieu Desnoyers
2019-08-16 19:15 ` Steven Rostedt
2019-08-17 14:27 ` Mathieu Desnoyers
2019-08-17 15:42 ` Steven Rostedt
2019-08-17 15:53 ` Mathieu Desnoyers
2019-08-17 16:43 ` Steven Rostedt
2019-08-16 12:32 ` WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16 12:41 ` Sebastian Andrzej Siewior
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