From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752663AbbGOBid (ORCPT ); Tue, 14 Jul 2015 21:38:33 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:45675 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752494AbbGOBia (ORCPT ); Tue, 14 Jul 2015 21:38:30 -0400 X-Helo: d03dlp02.boulder.ibm.com X-MailFrom: paulmck@linux.vnet.ibm.com X-RcptTo: linux-kernel@vger.kernel.org Date: Tue, 14 Jul 2015 18:38:20 -0700 From: "Paul E. McKenney" To: Will Deacon 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: <20150715013820.GA21971@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150713175029.GO2632@arm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150714193144.GP3717@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15071501-0005-0000-0000-000013994B64 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Thanx, Paul ------------------------------------------------------------------------ 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 */