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

[Steven Rostedt <rostedt@goodmis.org>]

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.

-- Steve


> 
> If coding style is your only concern here, we may want to consider
> introducing new macros in compiler.h:
> 
> WRITE_ONCE_INC(v) /* v++ */
> WRITE_ONCE_DEC(v) /* v-- */
> WRITE_ONCE_ADD(v, count) /* v += count */
> WRITE_ONCE_SUB(v, count) /* v -= count */
>