On Fri, Dec 11, 2015 at 02:06:49PM +0000, Will Deacon wrote: > On Fri, Dec 11, 2015 at 02:48:03PM +0100, Peter Zijlstra wrote: > > On Fri, Dec 11, 2015 at 01:33:14PM +0000, Will Deacon wrote: > > > On Fri, Dec 11, 2015 at 01:26:47PM +0100, Peter Zijlstra wrote: > > > > > > While we're there, the acquire in osq_wait_next() seems somewhat ill > > > > documented too. > > > > > > > > I _think_ we need ACQUIRE semantics there because we want to strictly > > > > order the lock-unqueue A,B,C steps and we get that with: > > > > > > > > A: SC > > > > B: ACQ > > > > C: Relaxed > > > > > > > > Similarly for unlock we want the WRITE_ONCE to happen after > > > > osq_wait_next, but in that case we can even rely on the control > > > > dependency there. > > > > > > Even for the lock-unqueue case, isn't B->C ordered by a control dependency > > > because C consists only of stores? > > > > Hmm, indeed. So we could go fully relaxed on it I suppose, since the > > same is true for the unlock site. > > In which case, we should be able to relax the xchg in there (osq_wait_next) > too, right?
Can I have second thoughts an confuse matters again? ;-) A RmW-acq is a load-acquire+store. That means the store is _after_ the load and thus not required for the completion of the control dependency. Therefore the store in question can reorder inside the conditional control block's stores. Hmm? Suggesting this acquire is in fact also wrong, since we need full order ops to guarantee full order both in lock and 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/
