On Mon, Aug 17, 2015 at 02:06:07PM +1000, Michael Ellerman wrote:

On Wed, 2015-08-12 at 08:43 -0700, Paul E. McKenney wrote:

quoted

On Wed, Aug 12, 2015 at 02:44:15PM +0100, Will Deacon wrote:

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Hello Paul,

On Fri, Jul 24, 2015 at 04:30:46PM +0100, Paul E. McKenney wrote:

quoted

On Fri, Jul 24, 2015 at 12:31:01PM +0100, Will Deacon wrote:

quoted

On Wed, Jul 15, 2015 at 02:12:21PM +0100, Paul E. McKenney wrote:

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quoted

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commit 695c05d4b9666c50b40a1c022678b5f6e2e3e771
Author: Paul E. McKenney [off-list ref]
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 [off-list ref]
    Cc: Will Deacon [off-list ref]
    Cc: Peter Zijlstra [off-list ref]
    Cc: Benjamin Herrenschmidt [off-list ref]
    Cc: "linux-arch@vger.kernel.org" [off-list ref]

Are you planning to queue this somewhere? I think it makes sense regardless
of whether we change PowerPc or not and ideally it would be merged around
the same time as my relaxed atomics series.

I have is in -rcu.  By default, I will push it to the 4.4 merge window.
Please let me know if you need it sooner.

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?

No it didn't.

I thought the end result of this thread was that we didn't *need* to change the
powerpc lock semantics? Or did I read it wrong?

ie. the docs now say that RELEASE+ACQUIRE is not a full barrier, which is
consistent with our current implementation.

That change happened about 1.5 years ago, and I thought that the
current discussion was about reversing it, based in part on the
recent powerpc benchmarks of locking primitives with and without the
sync instruction.  But regardless, I clearly cannot remove either the
smp_mb__after_unlock_lock() or the powerpc definition of it to be smp_mb()
if powerpc unlock/lock is not strengthened.

							Thanx, Paul