linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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Date:	Thu, 13 Aug 2015 06:10:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Thu, Aug 13, 2015 at 11:49:28AM +0100, Will Deacon wrote:
> On Wed, Aug 12, 2015 at 06:59:38PM +0100, Paul E. McKenney wrote:
> > On Wed, Aug 12, 2015 at 08:43:46AM -0700, Paul E. McKenney wrote:
> > > On Wed, Aug 12, 2015 at 02:44:15PM +0100, Will Deacon wrote:
> > > > The generic relaxed atomics are now queued in -tip, so it would be really
> > > > good to see this Documentation update land in 4.3 if at all possible. I
> > > > appreciate it's late in the cycle, but it's always worth asking.
> > > 
> > > Can't hurt to give it a try.  I have set -rcu's rcu/next branch to this
> > > commit, and if it passes a few day's worth of testing, I will see what
> > > Ingo has to say about a pull request.
> > > 
> > > This commit also privatizes smp_mb__after_unlock_lock() as well as
> > > updating documentation.  Looks like we need to strengthen powerpc's
> > > locking primitives, then get rid of smp_mb__after_unlock_lock() entirely.
> > > Or did that already happen and I just missed it?
> > 
> > And just for completeness, here is the current version of that commit.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> >  b/Documentation/memory-barriers.txt   |   71 +---------------------------------
> >  b/arch/powerpc/include/asm/spinlock.h |    2 
> >  b/include/linux/spinlock.h            |   10 ----
> >  b/kernel/rcu/tree.h                   |   12 +++++
> >  4 files changed, 16 insertions(+), 79 deletions(-)
> > 
> > commit 12d560f4ea87030667438a169912380be00cea4b
> > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Date:   Tue Jul 14 18:35:23 2015 -0700
> > 
> >     rcu,locking: Privatize smp_mb__after_unlock_lock()
> >     
> >     RCU is the only thing that uses smp_mb__after_unlock_lock(), and is
> >     likely the only thing that ever will use it, so this commit makes this
> >     macro private to RCU.
> >     
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >     Cc: Will Deacon <will.deacon@....com>
> >     Cc: Peter Zijlstra <peterz@...radead.org>
> >     Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> >     Cc: "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
> 
> Acked-by: Will Deacon <will.deacon@....com>
> 
> I don't think the PowerPC spinlock change is queued anywhere (I sent it
> out as a diff for discussion, but that was it). This patch doesn't rely
> on that though, right?

No, this patch just moves the smp_mb__after_unlock_lock() definition,
it does not change the code generated.

							Thanx, Paul

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