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

[Joel Fernandes]

On Fri, Aug 16, 2019 at 10:49:04PM +0200, Thomas Gleixner wrote:
> On Fri, 16 Aug 2019, Joel Fernandes wrote:
> > On Fri, Aug 16, 2019 at 3:19 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > > On Fri, 16 Aug 2019, Mathieu Desnoyers wrote:
> > >
> > > > If you choose not to use READ_ONCE(), then the "load tearing" issue can
> > > > cause similar spurious 1 -> 0 -> 1 transitions near 16-bit counter
> > > > overflow as described above. The "Invented load" also becomes an issue,
> > > > because the compiler could use the loaded value for a branch, and re-load
> > > > that value between two branches which are expected to use the same value,
> > > > effectively generating a corrupted state.
> > > >
> > > > I think we need a statement about whether READ_ONCE/WRITE_ONCE should
> > > > be used in this kind of situation, or if we are fine dealing with the
> > > > awkward compiler side-effects when they will occur.
> > >
> > > The only real downside (apart from readability) of READ_ONCE and
> > > WRITE_ONCE is that they prevent the compiler from optimizing accesses
> > > to the location being read or written.  But if you're just doing a
> > > single access in each place, not multiple accesses, then there's
> > > nothing to optimize anyway.  So there's no real reason not to use
> > > READ_ONCE or WRITE_ONCE.
> > 
> > I am also more on the side of using *_ONCE. To me, by principal, I
> > would be willing to convert any concurrent plain access using _ONCE,
> > just so we don't have to worry about it now or in the future and also
> > documents the access.
> 
> By that argumentation we need to plaster half of the kernel with _ONCE()
> and I'm so not looking forward to the insane amount of script kiddies
> patches to do that.

Really? That is quite scary that you are saying half of the kernel has issues
with concurrent access or compiler optimizations. It scares me that a
concurrent access can tear down a store/load and existing code can just fail,
if that is the case.

> Can we finally put a foot down and tell compiler and standard committee
> people to stop this insanity?

Sure, or could the compilers provide flags which prevent such optimization
similar to -O* flags?

thanks,

 - Joel