Messages in this thread | | | Subject | Re: spin_lock implicit/explicit memory barrier | From | Benjamin Herrenschmidt <> | Date | Thu, 11 Aug 2016 09:59:58 +1000 |
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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.
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