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Message-ID: <1450020330.317.11.camel@inria.fr>
Date: Sun, 13 Dec 2015 16:25:30 +0100
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: stdatomic implementation
Hello,
Am Sonntag, den 13.12.2015, 15:59 +0100 schrieb Szabolcs Nagy:
> * Jens Gustedt <jens.gustedt@...ia.fr> [2015-12-13 11:01:22 +0100]:
> > Its integration into musl should be straight forward and remains my
> > primary choice for having it distributed. You can find it at
> >
> > stdatomic.gforge.inria.fr
> >
>
> nice
>
> i recently run into a problem that you may be interested in:
> the gcc atomic builtins do not handle atomic pointer types as
> expected.
>
> T *p;
> __atomic_fetch_add(&p,1);
>
> adds 1 to (char*)p instead of sizeof(T).
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64843
thanks, I wasn't aware of that one.
FYI in a recent DR about atomics, among other things, I pointed on the
missing specification of atomic_fetch_add for pointer types. I think I
can safely say that the consensus among the committee for that
specific problem was to say that this was just an omission in
C11. Unfortunately, that DR opened a whole can of worms about atomics.
I was asked to cut it down in smaller pieces that will easier to
tackle with the DR process.
> > Under the hood it uses a new lock algorithm based on futex waits for
> > the slow path, that has proven to be more efficient than anything else
> > that I looked at: musl's internal lock, mtx_t or pthread_mutex_t, or
> > spinlocks. You can find a discussion of that at
> >
> > https://hal.inria.fr/hal-01236734
> >
> > Before I go (more) public with this, I would very much like to hear
> > what you think.
>
> i havent read the paper yet but it is worth noting that
> glibc implements weaker pthread semantics than musl, so
> on glibc pthread_mutex_lock/unlock is acquire/release.
> (which makes pthread_mutex_trylock non-conforming, but
> it's closer to the c11 semantics, same for spinlocks).
Hm, interesting. From our lengthy discussion, here, about mutexes and
conditions last year I had the vague impression that for musl also
this was more-or-less acquire/release, at least to completely seq_cst,
either.
> so it would be interesting to benchmark against glibc
> primitives too.
Yes, probably, also it would be interesting to have it on more
architectures and for more test cases.
Thanks
Jens
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