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Re: [PATCH][BZ 18743] PowerPC: Fix a race condition when eliding a lock
- From: Torvald Riegel <triegel at redhat dot com>
- To: Tulio Magno Quites Machado Filho <tuliom at linux dot vnet dot ibm dot com>
- Cc: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>, libc-alpha at sourceware dot org
- Date: Mon, 03 Aug 2015 18:21:09 +0200
- Subject: Re: [PATCH][BZ 18743] PowerPC: Fix a race condition when eliding a lock
- Authentication-results: sourceware.org; auth=none
- References: <1438274936-26493-1-git-send-email-tuliom at linux dot vnet dot ibm dot com> <55BA703D dot 7010303 at linaro dot org> <874mkl3wtq dot fsf at totoro dot lan>
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).