From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Fri, 11 Dec 2015 13:33:14 +0000 Subject: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX) In-Reply-To: <20151211122647.GM6356@twins.programming.kicks-ass.net> References: <5669D5F2.5050004@caviumnetworks.com> <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> Message-ID: <20151211133313.GG18828@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, Dec 11, 2015 at 01:26:47PM +0100, Peter Zijlstra wrote: > On Fri, Dec 11, 2015 at 12:18:00PM +0000, Will Deacon wrote: > > On Fri, Dec 11, 2015 at 01:13:19PM +0100, Peter Zijlstra wrote: > > > On Fri, Dec 11, 2015 at 12:04:19PM +0000, Will Deacon wrote: > > > > I think Andrew meant the atomic_xchg_acquire at the start of osq_lock, > > > > as opposed to "compare and swap". In which case, it does look like > > > > there's a bug here because there is nothing to order the initialisation > > > > of the node fields with publishing of the node, whether that's > > > > indirectly as a result of setting the tail to the current CPU or > > > > directly as a result of the WRITE_ONCE. > > > > > > Agreed, this does indeed look like a bug. If confirmed please write a > > > shiny changelog and I'll queue asap. > > > > Yup. I've failed to reproduce the issue locally, so we'll need to wait > > for Andrew and/or David to get back to us first. > > 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? Will