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Re: thoughts on libatomic
- From: Torvald Riegel <triegel at redhat dot com>
- To: Richard Henderson <rth at redhat dot com>
- Cc: Andrew MacLeod <amacleod at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 24 Apr 2012 20:24:21 +0200
- Subject: Re: thoughts on libatomic
- References: <email@example.com> <4F95ADAE.firstname.lastname@example.org>
On Mon, 2012-04-23 at 12:29 -0700, Richard Henderson wrote:
> On 04/16/12 11:15, Torvald Riegel wrote:
> > - for acq/rel memorders, we don't need seq_cst fences whenever the
> > atomicity is implemented with a lock
> > (hostconfig.c:pre_barrier/post_barrier)
> Err.. where? This also seems to conflict with...
I thought on the fallback paths that use locks. Locks typically use
acq/rel semantics, so for the particular location, we don't need
pre/post barrier code to be run.
Or maybe it didn't happen, and the one below is correct. Sorry for the
potential mix-up. I can look at this again after PTO.
> > - protect_start|end:
> > - BUG: those are called without additional post|pre_barrier calls in
> > cas_n.c, exch_n.c, fop_n.c, load_n.c, store_n.c
> > The lock membar guarantees are too weak if memorder for the atomic
> > access is seq_cst, and if you use more than one lock.
> > (assumes that lock implementation uses typical acq/rel barriers).
> ... this. I don't see what multiple locks has to do with seq vs acqrel.
If we don't run seq_cst membars when falling back to locks, then seq_cst
operations on two locations ordered by acq/rel locks will not be forced
to be in one total order (which seq_cst requires). They will be ordered
per lock, but not globally.
> > - config/posix/lock.c:
> > - I'm not sure that WATCH_SIZE should almost certainly be the same as
> > the cacheline size. Why? Which assumptions about the program?
> See above. Plus I assumed that if we have to take a lock, that's got to
> dominate over any cache ping-pong that would be played.
Perhaps the cache ping-pong dominates (and this assumes that the lock is
contended). But perhaps the cache ping-pong wouldn't be worse than the
contention potentially introduced from a too-coarse-grained lock?
> > - Do locks need to care about priority inversion?
> At the moment I'm leaving this with the pthread implementation.
pthreads offers PI-safe locks too. But I also don't know whether we