Re: [PATCH][BZ 18743] PowerPC: Fix a race condition when eliding a lock

[Torvald Riegel <triegel at redhat dot com>]

On Thu, 2015-07-30 at 23:06 -0300, Tulio Magno Quites Machado Filho
wrote:
> Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
> > So if the transaction fails it will just jump to else block.
> >
> >> +    else								\
> >> +      for (int i = __elision_aconf.try_tbegin; i > 0; i--)		\
> >> +	{								\
> >> +	  asm volatile("" ::: "memory");				\
> >
> > I can't really understand the requirement of this compiler barrier here.  If
> > compiler is moving around the 'is_lock_free' *before* __builtin_tbegin IMHO
> > this is a compiler bug.
> 
> This is the second problem and I agree with you again.  IMHO,  __builtin_tbegin
> starts a critical path and the compiler should not be moving a memory access out
> of the critical path or into the critical path.
> However, as the current implementations of GCC have this "issue" (some people
> may not agree with us), I believe we could carry this compiler barrier at least
> until all GCC versions supporting __builtin_tbegin are fixed.

I agree that a compiler should treat transaction begin / commit as
something like a lock acquisition / release, assuming that the
transaction semantics are equivalence to a single-global-lock semantics.

As far as glibc is concerned, I believe it's better to work around the
compiler issue by creating a glibc-internal wrapper for __builtin_tbegin
that enforces the semantics we believe it should have (eg, by adding the
compiler barrier).