Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()

From: Peter Zijlstra
Date: Thu Jan 16 2014 - 05:37:33 EST

On Thu, Jan 16, 2014 at 06:39:23AM +0700, Linus Torvalds wrote:
> On Jan 16, 2014 6:22 AM, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx> wrote:
> >
> > So while the primitive is called smp_store_release() the !SMP variant
> > still does:
> >
> > *(volatile __type *) = ptr;
> >
> > which should not compile on any Alpha pre EV56, SMP or no for __type ==
> > u8.
> I'm not sure where you get that "should not compile" theory from.
> I'm pretty sure it will compile just fine. It will just generate the same
> standard read-modify-write sequence (and not using the ldl/stc sequence
> either). Do you have any actual reason to believe it won't, apart from your
> theoretical wishes of how the world should work?

No, I earlier even said it probably would compile. My usage of 'should'
comes from how we've 'defined' volatile/ACCESS_ONCE in
Documentation/memory-barriers.txt. According to those constraints the
rmw cycle is not proper code.
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