linux-kernel - Re: spin_lock implicit/explicit memory barrier

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Date:	Thu, 11 Aug 2016 09:59:58 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Susanne Spraul <1vier1@....de>,
	Peter Zijlstra <peterz@...radead.org>, parri.andrea@...il.com
Subject: Re: spin_lock implicit/explicit memory barrier

On Wed, 2016-08-10 at 15:23 -0700, Davidlohr Bueso wrote:
> On Wed, 10 Aug 2016, Paul E. McKenney wrote:
> 
> > 
> > On Wed, Aug 10, 2016 at 08:21:22PM +0200, Manfred Spraul wrote:
>                                                               4)
> > > spin_unlock_wait() and spin_unlock() pair
> > > http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n291
> > > http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n409
> > > The data from the simple op must be observed by the following
> > > complex op. Right now, there is still an smp_rmb() in line 300: The
> > > control barrier from the loop inside spin_unlock_wait() is upgraded
> > > to an acquire barrier by an additional smp_rmb(). Is this smp_rmb()
> > > required? If I understand commit 2c6100227116 ("locking/qspinlock:
> > > Fix spin_unlock_wait() some more") right, with this commit qspinlock
> > > handle this case without the smp_rmb(). What I don't know if powerpc
> > > is using qspinlock already, or if powerpc works without the
> > > smp_rmb(). -- Manfred|
> 
> > No, ppc doesn't use qspinlocks, 

 ... yet. There are patches pending to add support for them

> but as mentioned, spin_unlock_wait for
> > tickets are now at least an acquire (ppc is stronger), 

The unlock path for qspinlock for us will be a release.

> which match that
> unlock store-release you are concerned about, this is as of 726328d92a4
> (locking/spinlock, arch: Update and fix spin_unlock_wait() implementations).
> 
> This is exactly what you are doing by upgrading the ctrl dependency to 
> the acquire barrier in http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n291
> and therefore we don't need it explicitly -- it also makes the comment
> wrt spin_unlock_wait obsolete. Or am I'm misunderstanding you?

Ben.