From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Fri, 11 Dec 2015 14:06:49 +0000 Subject: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX) In-Reply-To: <20151211134803.GP6356@twins.programming.kicks-ass.net> References: <20151211084133.GE6356@twins.programming.kicks-ass.net> <20151211120419.GD18828@arm.com> <20151211121319.GK6356@twins.programming.kicks-ass.net> <20151211121759.GE18828@arm.com> <20151211122647.GM6356@twins.programming.kicks-ass.net> <20151211133313.GG18828@arm.com> <20151211134803.GP6356@twins.programming.kicks-ass.net> Message-ID: <20151211140649.GI18828@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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? Will