Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

From: Peter Zijlstra <peterz@infradead.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
	mingo@kernel.org, linux-kernel@vger.kernel.org,
	paulmck@linux.vnet.ibm.com, corbet@lwn.net, mhocko@kernel.org,
	dhowells@redhat.com, torvalds@linux-foundation.org,
	will.deacon@arm.com, Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()
Date: Thu, 12 Nov 2015 11:28:12 +0100	[thread overview]
Message-ID: <20151112102812.GW17308@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20151112070915.GC6314@fixme-laptop.cn.ibm.com>

On Thu, Nov 12, 2015 at 03:14:51PM +0800, Boqun Feng wrote:
> Hmm.. probably incorrect.. because the ACQUIRE semantics of spin_lock()
> only guarantees that the memory operations following spin_lock() can't
> be reorder before the *LOAD* part of spin_lock() not the *STORE* part,
> i.e. the case below can happen(assuming the spin_lock() is implemented
> as ll/sc loop)
> 
> 	spin_lock(&lock):
> 	  r1 = *lock; // LL, r1 == 0
> 	o = READ_ONCE(object); // could be reordered here.
> 	  *lock = 1; // SC
> 
> This could happen because of the ACQUIRE semantics of spin_lock(), and 
> the current implementation of spin_lock() on PPC allows this happen.

Urgh, you just _had_ to send an email like this, didn't you ;-)

I think AARGH64 does the same thing. They either use LDAXR/STXR, which
also places the ACQUIRE on the load, or use LDADDA (v8.1) which I
suspect does the same, but I'm not entirely sure how to decode these 8.1
atomics yet.

Let me think a little more on this..