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

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Message-ID: <20160810222316.GA22336@linux-80c1.suse>
Date:	Wed, 10 Aug 2016 15:23:16 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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, 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, but as mentioned, spin_unlock_wait for
tickets are now at least an acquire (ppc is stronger), 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?

Thanks,
Davidlohr