Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753251AbbGOKvk (ORCPT ); Wed, 15 Jul 2015 06:51:40 -0400 Received: from foss.arm.com ([217.140.101.70]:38338 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752251AbbGOKvi (ORCPT ); Wed, 15 Jul 2015 06:51:38 -0400 Date: Wed, 15 Jul 2015 11:51:35 +0100 From: Will Deacon To: "Paul E. McKenney" Cc: Peter Zijlstra , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Benjamin Herrenschmidt Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Message-ID: <20150715105135.GE1005@arm.com> References: <20150713202032.GZ3717@linux.vnet.ibm.com> <20150713222346.GE19282@twins.programming.kicks-ass.net> <20150713230405.GB3717@linux.vnet.ibm.com> <20150714100429.GC15448@arm.com> <20150714124540.GC3717@linux.vnet.ibm.com> <20150714125146.GJ16213@arm.com> <20150714140014.GD3717@linux.vnet.ibm.com> <20150714141202.GN16213@arm.com> <20150714193144.GP3717@linux.vnet.ibm.com> <20150715013820.GA21971@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150715013820.GA21971@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 8633 Lines: 200 Hi Paul, On Wed, Jul 15, 2015 at 02:38:20AM +0100, Paul E. McKenney wrote: > On Tue, Jul 14, 2015 at 12:31:44PM -0700, Paul E. McKenney wrote: > > On Tue, Jul 14, 2015 at 03:12:16PM +0100, Will Deacon wrote: > > > On Tue, Jul 14, 2015 at 03:00:14PM +0100, Paul E. McKenney wrote: > > > > On Tue, Jul 14, 2015 at 01:51:46PM +0100, Will Deacon wrote: > > > > > On Tue, Jul 14, 2015 at 01:45:40PM +0100, Paul E. McKenney wrote: > > > > > > On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote: > > > > > > > Given that RCU is currently the only user of this barrier, how would you > > > > > > > feel about making the barrier local to RCU and not part of the general > > > > > > > memory-barrier API? > > > > > > > > > > > > In theory, no objection. Your thought is to leave the definitions where > > > > > > they are, mark them as being used only by RCU, and removing mention from > > > > > > memory-barriers.txt? Or did you have something else in mind? > > > > > > > > > > Actually, I was thinking of defining them in an RCU header file with an > > > > > #ifdef CONFIG_POWERPC for the smb_mb() version. Then you could have a big > > > > > comment describing the semantics, or put that in an RCU Documentation file > > > > > instead of memory-barriers.txt. > > > > > > > > > > That *should* then mean we notice anybody else trying to use the barrier, > > > > > because they'd need to send patches to either add something equivalent > > > > > or move the definition out again. > > > > > > > > My concern with this approach is that someone putting together a new > > > > architecture might miss this. That said, this approach certainly would > > > > work for the current architectures. > > > > > > I don't think they're any more likely to miss it than with the current > > > situation where the generic code defines the macro as a NOP unless you > > > explicitly override it. > > > > Fair enough... > > Like this? Precisely! Thanks for cooking the patch -- this lays all my worries to rest, so: Acked-by: Will Deacon We should continue the discussion with Ben and Michael about whether or not the PowerPC locking code can be strengthened, though (making this barrier a NOP on all currently supported archs). Will > commit 695c05d4b9666c50b40a1c022678b5f6e2e3e771 > Author: Paul E. McKenney > Date: Tue Jul 14 18:35:23 2015 -0700 > > rcu,locking: Privatize smp_mb__after_unlock_lock() > > RCU is the only thing that uses smp_mb__after_unlock_lock(), and is > likely the only thing that ever will use it, so this commit makes this > macro private to RCU. > > Signed-off-by: Paul E. McKenney > Cc: Will Deacon > Cc: Peter Zijlstra > Cc: Benjamin Herrenschmidt > Cc: "linux-arch@vger.kernel.org" > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index 318523872db5..eafa6a53f72c 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -1854,16 +1854,10 @@ RELEASE are to the same lock variable, but only from the perspective of > another CPU not holding that lock. In short, a ACQUIRE followed by an > RELEASE may -not- be assumed to be a full memory barrier. > > -Similarly, the reverse case of a RELEASE followed by an ACQUIRE does not > -imply a full memory barrier. If it is necessary for a RELEASE-ACQUIRE > -pair to produce a full barrier, the ACQUIRE can be followed by an > -smp_mb__after_unlock_lock() invocation. This will produce a full barrier > -(including transitivity) if either (a) the RELEASE and the ACQUIRE are > -executed by the same CPU or task, or (b) the RELEASE and ACQUIRE act on > -the same variable. The smp_mb__after_unlock_lock() primitive is free > -on many architectures. Without smp_mb__after_unlock_lock(), the CPU's > -execution of the critical sections corresponding to the RELEASE and the > -ACQUIRE can cross, so that: > +Similarly, the reverse case of a RELEASE followed by an ACQUIRE does > +not imply a full memory barrier. Therefore, the CPU's execution of the > +critical sections corresponding to the RELEASE and the ACQUIRE can cross, > +so that: > > *A = a; > RELEASE M > @@ -1901,29 +1895,6 @@ the RELEASE would simply complete, thereby avoiding the deadlock. > a sleep-unlock race, but the locking primitive needs to resolve > such races properly in any case. > > -With smp_mb__after_unlock_lock(), the two critical sections cannot overlap. > -For example, with the following code, the store to *A will always be > -seen by other CPUs before the store to *B: > - > - *A = a; > - RELEASE M > - ACQUIRE N > - smp_mb__after_unlock_lock(); > - *B = b; > - > -The operations will always occur in one of the following orders: > - > - STORE *A, RELEASE, ACQUIRE, smp_mb__after_unlock_lock(), STORE *B > - STORE *A, ACQUIRE, RELEASE, smp_mb__after_unlock_lock(), STORE *B > - ACQUIRE, STORE *A, RELEASE, smp_mb__after_unlock_lock(), STORE *B > - > -If the RELEASE and ACQUIRE were instead both operating on the same lock > -variable, only the first of these alternatives can occur. In addition, > -the more strongly ordered systems may rule out some of the above orders. > -But in any case, as noted earlier, the smp_mb__after_unlock_lock() > -ensures that the store to *A will always be seen as happening before > -the store to *B. > - > Locks and semaphores may not provide any guarantee of ordering on UP compiled > systems, and so cannot be counted on in such a situation to actually achieve > anything at all - especially with respect to I/O accesses - unless combined > @@ -2154,40 +2125,6 @@ But it won't see any of: > *E, *F or *G following RELEASE Q > > > -However, if the following occurs: > - > - CPU 1 CPU 2 > - =============================== =============================== > - WRITE_ONCE(*A, a); > - ACQUIRE M [1] > - WRITE_ONCE(*B, b); > - WRITE_ONCE(*C, c); > - RELEASE M [1] > - WRITE_ONCE(*D, d); WRITE_ONCE(*E, e); > - ACQUIRE M [2] > - smp_mb__after_unlock_lock(); > - WRITE_ONCE(*F, f); > - WRITE_ONCE(*G, g); > - RELEASE M [2] > - WRITE_ONCE(*H, h); > - > -CPU 3 might see: > - > - *E, ACQUIRE M [1], *C, *B, *A, RELEASE M [1], > - ACQUIRE M [2], *H, *F, *G, RELEASE M [2], *D > - > -But assuming CPU 1 gets the lock first, CPU 3 won't see any of: > - > - *B, *C, *D, *F, *G or *H preceding ACQUIRE M [1] > - *A, *B or *C following RELEASE M [1] > - *F, *G or *H preceding ACQUIRE M [2] > - *A, *B, *C, *E, *F or *G following RELEASE M [2] > - > -Note that the smp_mb__after_unlock_lock() is critically important > -here: Without it CPU 3 might see some of the above orderings. > -Without smp_mb__after_unlock_lock(), the accesses are not guaranteed > -to be seen in order unless CPU 3 holds lock M. > - > > ACQUIRES VS I/O ACCESSES > ------------------------ > diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h > index 4dbe072eecbe..523673d7583c 100644 > --- a/arch/powerpc/include/asm/spinlock.h > +++ b/arch/powerpc/include/asm/spinlock.h > @@ -28,8 +28,6 @@ > #include > #include > > -#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */ > - > #ifdef CONFIG_PPC64 > /* use 0x800000yy when locked, where yy == CPU number */ > #ifdef __BIG_ENDIAN__ > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index 80d974df0ea0..a9fea7395ba2 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -645,3 +645,15 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll) > #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */ > } > #endif /* #ifdef CONFIG_RCU_TRACE */ > + > +/* > + * Place this after a lock-acquisition primitive to guarantee that > + * an UNLOCK+LOCK pair act as a full barrier. This guarantee applies > + * if the UNLOCK and LOCK are executed by the same CPU or if the > + * UNLOCK and LOCK operate on the same lock variable. > + */ > +#ifdef CONFIG_PPC > +#define smp_mb__after_unlock_lock() smp_mb() /* Full ordering for lock. */ > +#else /* #ifdef CONFIG_PPC */ > +#define smp_mb__after_unlock_lock() do { } while (0) > +#endif /* #else #ifdef CONFIG_PPC */ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/