From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Duyck <alexander.h.duyck@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>,
"alexander.duyck@gmail.com" <alexander.duyck@gmail.com>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Michael Neuling <mikey@neuling.org>,
Tony Luck <tony.luck@intel.com>,
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Oleg Nesterov <oleg@redhat.com>,
Michael Ellerman <michael@ellerman.id.au>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Martin Schwidefsky <schwidefsky@de.ibm.com>,
Russell King <linux@arm.linux.org.uk>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] arch: Introduce read_acquire()
Date: Wed, 12 Nov 2014 16:37:40 +0100 [thread overview]
Message-ID: <20141112153740.GK29390@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <54637B6A.9070204@redhat.com>
On Wed, Nov 12, 2014 at 07:23:22AM -0800, Alexander Duyck wrote:
>
> On 11/12/2014 02:15 AM, Peter Zijlstra wrote:
> >On Tue, Nov 11, 2014 at 01:12:32PM -0800, Alexander Duyck wrote:
> >>>Minor nit on naming, but load_acquire would match what we do with barriers,
> >>>where you simply drop the smp_ prefix if you want the thing to work on UP
> >>>systems too.
> >>The problem is this is slightly different, load_acquire in my mind would use
> >>a mb() call, I only use a rmb(). That is why I chose read_acquire as the
> >>name.
> >acquire is not about rmb vs mb, do read up on
> >Documentation/memory-barriers.txt. Its a distinctly different semantic.
> >Some archs simply lack the means of implementing this semantics and have
> >to revert to mb (stronger is always allowed).
> >
> >Using the read vs load to wreck the acquire semantics is just insane.
>
> Actually I have been reading up on it as I wasn't familiar with C11.
C11 is _different_ although somewhat related.
> Most
> of what I was doing was actually based on the documentation in barriers.txt
> which was referring to memory operations not loads/stores when referring to
> the acquire/release so I assumed the full memory barrier was required. I
> wasn't aware that smp_load_acquire was only supposed to be ordering loads,
> or that smp_ store_release only applied to stores.
It does not.. an ACQUIRE is a semi-permeable barrier that doesn't allow
LOADs nor STOREs that are issued _after_ it to appear to happen _before_.
The RELEASE is the opposite number, it ensures LOADs and STOREs that are
issued _before_ cannot happen _after_.
This typically matches locking, where a lock (mutex_lock, spin_lock
etc..) have ACQUIRE semantics and the unlock RELEASE. Such that:
spin_lock();
a = 1;
b = x;
spin_unlock();
guarantees all LOADs (x) and STORESs (a,b) happen _inside_ the lock
region. What they do not guarantee is:
y = 1;
spin_lock()
a = 1;
b = x;
spin_unlock()
z = 4;
An order between y and z, both are allowed _into_ the region and can
cross there like:
spin_lock();
...
z = 4;
y = 1;
...
spin_unlock();
The only 'open' issue at the moment is if RELEASE+ACQUIRE := MB.
Currently we say this is not so, but Will (and me) would very much like
this to be so -- PPC64 being the only arch that actually makes this
distinction.
next prev parent reply other threads:[~2014-11-12 15:38 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-11 18:57 [PATCH] arch: Introduce read_acquire() alexander.duyck
2014-11-11 19:40 ` Linus Torvalds
2014-11-11 20:45 ` Alexander Duyck
2014-11-12 10:10 ` Peter Zijlstra
2014-11-12 10:10 ` Will Deacon
2014-11-12 15:42 ` Alexander Duyck
2014-11-11 19:47 ` Will Deacon
2014-11-11 21:12 ` Alexander Duyck
2014-11-12 10:15 ` Peter Zijlstra
2014-11-12 15:23 ` Alexander Duyck
2014-11-12 15:37 ` Peter Zijlstra [this message]
2014-11-12 19:24 ` Alexander Duyck
2014-11-12 20:43 ` David Miller
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