Messages in this thread | | | Date | Tue, 17 Nov 2015 13:01:09 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() |
| |
On Tue, Nov 17, 2015 at 11:51:10AM +0000, Will Deacon wrote: > Hi Linus, > > On Mon, Nov 16, 2015 at 01:58:49PM -0800, Linus Torvalds wrote: > > On Mon, Nov 16, 2015 at 8:24 AM, Will Deacon <will.deacon@arm.com> wrote: > > > > > > ... or we upgrade spin_unlock_wait to a LOCK operation, which might be > > > slightly cheaper than spin_lock()+spin_unlock(). > > > > So traditionally the real concern has been the cacheline ping-pong > > part of spin_unlock_wait(). I think adding a memory barrier (that > > doesn't force any exclusive states, just ordering) to it is fine, but > > I don't think we want to necessarily have it have to get the cacheline > > into exclusive state. > > The problem is, I don't think the memory-barrier buys you anything in > the context of Boqun's example. In fact, he already had smp_mb() either > side of the spin_unlock_wait() and its still broken on arm64 and ppc. > > Paul is proposing adding a memory barrier after spin_lock() in the racing > thread, but I personally think people will forget to add that.
A mechanical check would certainly make me feel better about it, so that any lock that was passed to spin_unlock_wait() was required to have all acquisitions followed by smp_mb__after_unlock_lock() or some such. But I haven't yet given up on finding a better solution.
Thanx, Paul
| |