On Mon, Nov 16, 2015 at 04:46:36PM +0000, Will Deacon wrote: > On Mon, Nov 16, 2015 at 08:44:43AM -0800, Paul E. McKenney wrote: > > On Mon, Nov 16, 2015 at 04:24:53PM +0000, Will Deacon wrote: > > > On Mon, Nov 16, 2015 at 05:04:45PM +0100, Peter Zijlstra wrote: > > > > On Mon, Nov 16, 2015 at 04:56:58PM +0100, Peter Zijlstra wrote: > > > > > On Thu, Nov 12, 2015 at 10:21:39AM -0800, Linus Torvalds wrote: > > > > > > Now, the point of spin_unlock_wait() (and "spin_is_locked()") should > > > > > > generally be that you have some external ordering guarantee that > > > > > > guarantees that the lock has been taken. For example, for the IPC > > > > > > semaphores, we do either one of: > > > > > > > > > > > > (a) get large lock, then - once you hold that lock - wait for each > > > > > > small lock > > > > > > > > > > > > or > > > > > > > > > > > > (b) get small lock, then - once you hold that lock - check that the > > > > > > largo lock is unlocked > > > > > > > > > > > > and that's the case we should really worry about. The other uses of > > > > > > spin_unlock_wait() should have similar "I have other reasons to know > > > > > > I've seen that the lock was taken, or will never be taken after this > > > > > > because XYZ". > > > > > > > > > > I don't think this is true for the usage in do_exit(), we have no > > > > > knowledge on if pi_lock is taken or not. We just want to make sure > > > > > that > > > > > _if_ it were taken, we wait until it is released. > > > > > > > > And unless PPC would move to using RCsc locks with a SYNC in > > > > spin_lock(), I don't think it makes sense to add > > > > smp_mb__after_unlock_lock() to all tsk->pi_lock instances to fix this. > > > > As that is far more expensive than flipping the exit path to do > > > > spin_lock()+spin_unlock(). > > > > > > ... or we upgrade spin_unlock_wait to a LOCK operation, which might be > > > slightly cheaper than spin_lock()+spin_unlock(). > > > > Or we supply a heavyweight version of spin_unlock_wait() that forces > > the cache miss. But I bet that the difference in overhead between > > spin_lock()+spin_unlock() and the heavyweight version would be down in > > the noise. > > I'm not so sure. If the lock is ticket-based, then spin_lock() has to > queue for its turn, whereas spin_unlock_wait could just wait for the > next unlock.
Fair point, and it actually applies to high-contention spinlocks as well, just a bit less deterministically. OK, given that I believe that we do see high contention on the lock in question, I withdraw any objections to a heavy-weight form of spin_unlock_wait(). Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/