linux-kernel - Re: [PATCHv2 06/11] atomics/treewide: rework ordering barriers

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Message-ID: <20180705101241.7q7nvmzkfsanpnbr@lakrids.cambridge.arm.com>
Date:   Thu, 5 Jul 2018 11:12:41 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        boqun.feng@...il.com, Andrea Parri <parri.andrea@...il.com>
Subject: Re: [PATCHv2 06/11] atomics/treewide: rework ordering barriers

On Wed, Jul 04, 2018 at 06:50:00PM +0100, Will Deacon wrote:
> On Wed, Jul 04, 2018 at 04:56:19PM +0100, Mark Rutland wrote:
> > On Wed, Jul 04, 2018 at 04:06:46PM +0100, Will Deacon wrote:
> > > On Mon, Jun 25, 2018 at 11:59:47AM +0100, Mark Rutland wrote:
> > > > Currently architectures can override __atomic_op_*() to define the barriers
> > > > used before/after a relaxed atomic when used to build acquire/release/fence
> > > > variants.
> > > > 
> > > > This has the unfortunate property of requiring the architecture to define the
> > > > full wrapper for the atomics, rather than just the barriers they care about,
> > > > and gets in the way of generating atomics which can be easily read.
> > > > 
> > > > Instead, this patch has architectures define an optional set of barriers,
> > > > __atomic_mb_{before,after}_{acquire,release,fence}(), which <linux/atomic.h>
> > > > uses to build the wrappers.
> > > 
> > > Looks like you've renamed these in the patch but not updated the commit
> > > message.
> > 
> > Yup; Peter also pointed that out. In my branch this now looks like:
> > 
> > ----
> > Instead, this patch has architectures define an optional set of barriers:
> > 
> > * __atomic_acquire_fence()
> > * __atomic_release_fence()
> > * __atomic_pre_fence()
> > * __atomic_post_fence()
> > 
> > ... which <linux/atomic.h> uses to build the wrappers.
> > ----
> > 
> > ... which is hopefully more legible, too!
> > 
> > > Also, to add to the bikeshedding, would it worth adding "rmw" in there
> > > somewhere, e.g. __atomic_post_rmw_fence, since I assume these only
> > > apply to value-returning stuff?
> > 
> > I don't have any opinion there, but I'm also not sure I've parsed your
> > rationale correctly. I guess a !RMW full-fence op doesn't make sense? Or
> > that's something we want to avoid in the API?
> > 
> > AFAICT, we only use __atomic_{pre,post}_fence() for RMW ops today.
> 
> No, I think you're right and my terminology is confused. Leave it as-is
> for the moment.

Sure thing.

Perhaps __atomic_{pre,post}_full_fence() might be better, assuming
you're trying to avoid people erroneously assuming that
__atomic_{pre,post}_fence() are like acquire/release fences.

Thanks,
Mark.