On Fri, Dec 11, 2015 at 02:35:40PM -0800, Paul E. McKenney 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. > > I am probably missing quite a bit on this thread, but don't x86 MMIO > accesses to frame buffers need to interact with something more heavyweight > than an x86 release store or acquire load in order to remain confined > to the resulting critical section?
So on x86 there really isn't a problem because every atomic op (and there's plenty here) will be a full barrier. That is, even if you were to replace everything with _relaxed() ops, it would still work as 'expected' on x86. ppc/arm64 will crash and burn, but that's another story. But the important point here was that osq_wait_next() is never relied upon to provide either the ACQUIRE semantics for osq_lock() not the RELEASE semantics for osq_unlock(). Those are provided by other ops. -- 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/