linux-kernel - Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

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Date:   Sat, 17 Aug 2019 11:42:18 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On Sat, 17 Aug 2019 10:27:39 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> 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.

Note, this is done at every sched switch, for both next and prev tasks.
And the update is only done at the enabling of a tracepoint (very rare
occurrence) If it missed it scheduling in, it has a really good chance
of getting it while scheduling out.

And 99.999% of my tracing that I do, the tasks scheduling in when
enabling a tracepoint is not what I even care about, as I enable
tracing then start what I want to trace.


> 
> 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.

I'm more OK with using a READ_ONCE() on the flags so it is consistent.
But the WRITE_ONCE() is going a bit overboard.

> 
> 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.

Which part of the code are you talking about here?

-- Steve