From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rostedt <rostedt@goodmis.org>
Cc: 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: Sat, 17 Aug 2019 10:27:39 -0400 (EDT) [thread overview]
Message-ID: <621310481.23873.1566052059389.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20190816151541.6864ff30@oasis.local.home>
----- On Aug 16, 2019, at 3:15 PM, rostedt rostedt@goodmis.org wrote:
> On Fri, 16 Aug 2019 13:19:20 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>
>> ----- On Aug 16, 2019, at 12:25 PM, rostedt rostedt@goodmis.org wrote:
>>
>> > On Fri, 16 Aug 2019 10:26:43 -0400 Mathieu Desnoyers
>> > <mathieu.desnoyers@efficios.com> 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.
>>
>> The update is done while holding the mutex, but the read-side does not
>> hold that mutex, so it can observe the intermediate state caused by
>> store-tearing or invented stores which can be generated by the compiler
>> on the update-side.
>>
>> Please refer to the following LWN article:
>>
>> https://lwn.net/Articles/793253/
>>
>> Sections:
>> - "Store tearing"
>> - "Invented stores"
>>
>> Arguably, based on that article, store tearing is only observed in the
>> wild for constants (which is not the case here), and invented stores
>> seem to require specific code patterns. But I wonder why we would ever want to
>> pair a fragile non-volatile store with a READ_ONCE() ? Considering the pain
>> associated to reproduce and hunt down this kind of issue in the wild, I would
>> be tempted to enforce that any READ_ONCE() operating on a variable would either
>> need to be paired with WRITE_ONCE() or with atomic operations, so those can
>> eventually be validated by static code checkers and code sanitizers.
>
> My issue is that this is just a case to decide if we should cache a
> comm or not. It's a helper, nothing more. There's no guarantee that
> something will get cached.
I get your point wrt WRITE_ONCE(): since it's a cache it should not have
user-visible effects if a temporary incorrect value is observed. Well in
reality, it's not a cache: if the lookup fails, it returns "<...>" instead,
so cache lookup failure ends up not providing any useful data in the trace.
Let's assume this is a known and documented tracer limitation.
However, wrt READ_ONCE(), things are different. The variable read ends up
being used to control various branches in the code, and the compiler could
decide to re-fetch the variable (with a different state), and therefore
cause _some_ of the branches to be inconsistent. See
tracing_record_taskinfo_sched_switch() and tracing_record_taskinfo() @flags
parameter.
AFAIU the current code should not generate any out-of-bound writes in case of
re-fetch, but no comment in there documents how fragile this is.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2019-08-17 14:27 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
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 [this message]
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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