From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Date: Mon, 18 Jan 2016 15:46:29 +0000 Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Message-Id: <20160118154629.GB3818@linux.vnet.ibm.com> List-Id: References: <20160114204827.GE3818@linux.vnet.ibm.com> <20160118081929.GA30420@gondor.apana.org.au> In-Reply-To: <20160118081929.GA30420@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Herbert Xu Cc: linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, mst@redhat.com, peterz@infradead.org, will.deacon@arm.com, virtualization@lists.linux-foundation.org, hpa@zytor.com, sparclinux@vger.kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, linux@arm.linux.org.uk, arnd@arndb.de, linux-sh@vger.kernel.org, mpe@ellerman.id.au, x86@kernel.org, Linus Torvalds , xen-devel@lists.xenproject.org, mingo@elte.hu, linux-xtensa@linux-xtensa.org, james.hogan@imgtec.com, user-mode-linux-devel@lists.sourceforge.net, stefano.stabellini@eu.citrix.com, adi-buildroot-devel@lists.sourceforge.net, Leonid.Yegoshin@imgtec.com, ddaney.cavm@gmail.com, tglx@linutronix.de, linux-metag@vger.kernel.org, linux-arm-kernel@lists.infradead.org, andrew.cooper3@citrix.com, linux-kernel@vger.kernel.org, ralf@linux-mips.org, joe@perches.com On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote: > Paul E. McKenney wrote: > > > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > The reason for this is that smp_read_barrier_depends() must order the > > pointer load against any subsequent read or write through a dereference > > of that pointer. For example: > > > > p = READ_ONCE(gp); > > smp_rmb(); > > r1 = p->a; /* ordered by smp_rmb(). */ > > p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */ > > r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */ > > > > In contrast: > > > > p = READ_ONCE(gp); > > smp_read_barrier_depends(); > > r1 = p->a; /* ordered by smp_read_barrier_depends(). */ > > p->b = 42; /* ordered by smp_read_barrier_depends(). */ > > r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */ > > > > Again, if your hardware maintains local ordering for address > > and data dependencies, you can have read_barrier_depends() and > > smp_read_barrier_depends() be no-ops like they are for most > > architectures. > > > > Does that help? > > This is crazy! smp_rmb started out being strictly stronger than > smp_read_barrier_depends, when did this stop being the case? Hello, Herbert! It is true that most Linux kernel code relies only on the read-read properties of dependencies, but the read-write properties are useful. Admittedly relatively rarely, but useful. The better comparison for smp_read_barrier_depends(), especially in its rcu_dereference*() form, is smp_load_acquire(). Thanx, Paul From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755595AbcARPqu (ORCPT ); Mon, 18 Jan 2016 10:46:50 -0500 Received: from e31.co.us.ibm.com ([32.97.110.149]:32922 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755533AbcARPqq (ORCPT ); Mon, 18 Jan 2016 10:46:46 -0500 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-arch@vger.kernel.org;linux-ia64@vger.kernel.org;linux-kernel@vger.kernel.org;linux-metag@vger.kernel.org;linux-s390@vger.kernel.org;linux-sh@vger.kernel.org;sparclinux@vger.kernel.org Date: Mon, 18 Jan 2016 07:46:29 -0800 From: "Paul E. McKenney" To: Herbert Xu Cc: Leonid.Yegoshin@imgtec.com, linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, mst@redhat.com, peterz@infradead.org, will.deacon@arm.com, virtualization@lists.linux-foundation.org, hpa@zytor.com, sparclinux@vger.kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, linux@arm.linux.org.uk, user-mode-linux-devel@lists.sourceforge.net, linux-sh@vger.kernel.org, mpe@ellerman.id.au, x86@kernel.org, xen-devel@lists.xenproject.org, mingo@elte.hu, linux-xtensa@linux-xtensa.org, james.hogan@imgtec.com, arnd@arndb.de, stefano.stabellini@eu.citrix.com, adi-buildroot-devel@lists.sourceforge.net, ddaney.cavm@gmail.com, tglx@linutronix.de, linux-metag@vger.kernel.org, linux-arm-kernel@lists.infradead.org, andrew.cooper3@citrix.com, linux-kernel@vger.kernel.org, ralf@linux-mips.org, joe@perches.com, linuxppc-dev@lists.ozlabs.org, davem@davemloft.net, Linus Torvalds Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Message-ID: <20160118154629.GB3818@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20160114204827.GE3818@linux.vnet.ibm.com> <20160118081929.GA30420@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160118081929.GA30420@gondor.apana.org.au> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16011815-8236-0000-0000-00001548EE43 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote: > Paul E. McKenney wrote: > > > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > The reason for this is that smp_read_barrier_depends() must order the > > pointer load against any subsequent read or write through a dereference > > of that pointer. For example: > > > > p = READ_ONCE(gp); > > smp_rmb(); > > r1 = p->a; /* ordered by smp_rmb(). */ > > p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */ > > r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */ > > > > In contrast: > > > > p = READ_ONCE(gp); > > smp_read_barrier_depends(); > > r1 = p->a; /* ordered by smp_read_barrier_depends(). */ > > p->b = 42; /* ordered by smp_read_barrier_depends(). */ > > r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */ > > > > Again, if your hardware maintains local ordering for address > > and data dependencies, you can have read_barrier_depends() and > > smp_read_barrier_depends() be no-ops like they are for most > > architectures. > > > > Does that help? > > This is crazy! smp_rmb started out being strictly stronger than > smp_read_barrier_depends, when did this stop being the case? Hello, Herbert! It is true that most Linux kernel code relies only on the read-read properties of dependencies, but the read-write properties are useful. Admittedly relatively rarely, but useful. The better comparison for smp_read_barrier_depends(), especially in its rcu_dereference*() form, is smp_load_acquire(). Thanx, Paul From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Date: Mon, 18 Jan 2016 07:46:29 -0800 Message-ID: <20160118154629.GB3818@linux.vnet.ibm.com> References: <20160114204827.GE3818@linux.vnet.ibm.com> <20160118081929.GA30420@gondor.apana.org.au> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20160118081929.GA30420@gondor.apana.org.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Archive: List-Post: To: Herbert Xu Cc: linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, mst@redhat.com, peterz@infradead.org, will.deacon@arm.com, virtualization@lists.linux-foundation.org, hpa@zytor.com, sparclinux@vger.kernel.org, mingo@kernel.org, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, linux@arm.linux.org.uk, arnd@arndb.de, linux-sh@vger.kernel.org, mpe@ellerman.id.au, x86@kernel.org, Linus Torvalds , xen-devel@lists.xenproject.org, mingo@elte.hu, linux-xtensa@linux-xtensa.org, james.hogan@imgtec.com, user-mode-linux-devel@lists.sourceforge.net, stefano.stabellini@eu.citrix.com, adi-buildroot-devel@lists.sourceforge.net, Leonid.Yegoshin@imgtec.com, ddaney.cavm@gmail.com, tglx@linutronix.de, linux-metag@vger.kernel.org, linux-arm-kernel@lists.infradead.org, andrew.cooper3@citrix.com, linux-kernel@vger.kernel.org, ralf@linux-mips.org, joe@perches.com List-ID: On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote: > Paul E. McKenney wrote: > > > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > The reason for this is that smp_read_barrier_depends() must order the > > pointer load against any subsequent read or write through a dereference > > of that pointer. For example: > > > > p = READ_ONCE(gp); > > smp_rmb(); > > r1 = p->a; /* ordered by smp_rmb(). */ > > p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */ > > r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */ > > > > In contrast: > > > > p = READ_ONCE(gp); > > smp_read_barrier_depends(); > > r1 = p->a; /* ordered by smp_read_barrier_depends(). */ > > p->b = 42; /* ordered by smp_read_barrier_depends(). */ > > r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */ > > > > Again, if your hardware maintains local ordering for address > > and data dependencies, you can have read_barrier_depends() and > > smp_read_barrier_depends() be no-ops like they are for most > > architectures. > > > > Does that help? > > This is crazy! smp_rmb started out being strictly stronger than > smp_read_barrier_depends, when did this stop being the case? Hello, Herbert! It is true that most Linux kernel code relies only on the read-read properties of dependencies, but the read-write properties are useful. Admittedly relatively rarely, but useful. The better comparison for smp_read_barrier_depends(), especially in its rcu_dereference*() form, is smp_load_acquire(). Thanx, Paul From mboxrd@z Thu Jan 1 00:00:00 1970 From: paulmck@linux.vnet.ibm.com (Paul E. McKenney) Date: Mon, 18 Jan 2016 07:46:29 -0800 Subject: [v3,11/41] mips: reuse asm-generic/barrier.h In-Reply-To: <20160118081929.GA30420@gondor.apana.org.au> References: <20160114204827.GE3818@linux.vnet.ibm.com> <20160118081929.GA30420@gondor.apana.org.au> Message-ID: <20160118154629.GB3818@linux.vnet.ibm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Mon, Jan 18, 2016 at 04:19:29PM +0800, Herbert Xu wrote: > Paul E. McKenney wrote: > > > > You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > The reason for this is that smp_read_barrier_depends() must order the > > pointer load against any subsequent read or write through a dereference > > of that pointer. For example: > > > > p = READ_ONCE(gp); > > smp_rmb(); > > r1 = p->a; /* ordered by smp_rmb(). */ > > p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */ > > r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */ > > > > In contrast: > > > > p = READ_ONCE(gp); > > smp_read_barrier_depends(); > > r1 = p->a; /* ordered by smp_read_barrier_depends(). */ > > p->b = 42; /* ordered by smp_read_barrier_depends(). */ > > r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */ > > > > Again, if your hardware maintains local ordering for address > > and data dependencies, you can have read_barrier_depends() and > > smp_read_barrier_depends() be no-ops like they are for most > > architectures. > > > > Does that help? > > This is crazy! smp_rmb started out being strictly stronger than > smp_read_barrier_depends, when did this stop being the case? Hello, Herbert! It is true that most Linux kernel code relies only on the read-read properties of dependencies, but the read-write properties are useful. Admittedly relatively rarely, but useful. The better comparison for smp_read_barrier_depends(), especially in its rcu_dereference*() form, is smp_load_acquire(). Thanx, Paul