Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()

Paul E. McKenney
2015-06-17T18:02:42Z

On Wed, Jun 17, 2015 at 07:11:40PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 17, 2015 at 09:37:31AM -0700, Paul E. McKenney wrote:
> > The point of std::atomic<> (and of the equivalent C11 syntax) is to
> > force the compiler to suppress optimizations that are unsafe for shared
> > variables. We get more or less the same effect with volatile, protests
> > from compiler people notwithstanding.
> >
> > I often tell the compiler guys that they have to expect make -some-
> > concessions for being 30 years late to the concurrency party, but
> > it nevertheless makes sense to future-proof our code where it is
> > reasonable to do so.
>
> Right, so in that regards I would request the compiler option (and or
> #pragma) that disables all the out-of-thin-air nonsense.

OK. What is the form of the #pragma? If it focuses on a specific
access, we are likely to get a lot of pushback.

> Because while they hide behind their undefined behaviour, the fact is
> that all of their machines for the past 30 odd years have been relying
> on this 'undefined' behaviour to work. This being the machines they've
> been typing their useless specs on :-)

Maybe I can scare them into doing all their work on UP systems. ;-)

Interestingly enough, LLVM is taking a slightly different approach.
Rather than invoke undefined behavior, they say that data races result
in random bits being loaded. Not that it makes much difference to the
health and well-being of the software, mind you...

> I doubt there's a single OS kernel (that supports SMP configurations)
> that does not rely on a whole host of 'undefined' behaviour.

An alternative approach would be a compiler switch (or similar) that
changed the default atomic access from SC to relaxed. Then shared
variables could be marked atomic, and normal C code could be used to
access them, but without the compiler emitting memory barriers all over
the place (yes, even on x86).

Thanx, Paul