Messages in this thread | | | Date | Mon, 16 Nov 2015 16:24:53 +0000 | From | Will Deacon <> | Subject | Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() |
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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().
Will
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