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

[Steven Rostedt <rostedt@goodmis.org>]

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