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(). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
