Re: [PATCH v8 4/4] qrwlock: Use smp_store_release() in write_unlock()
From: Paul E. McKenney
Date: Sat Jan 18 2014 - 05:01:28 EST
On Thu, Jan 16, 2014 at 11:36:59AM +0100, Peter Zijlstra wrote:
> 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.
OK, I will bite... Aside from fine-grained code timing, what code could
you write to tell the difference between a real one-byte store and an
RMW emulating that store?
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/