Messages in this thread | | | Date | Sat, 17 Aug 2019 11:42:18 -0400 | From | Steven Rostedt <> | 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@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
| |