From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Date: Tue, 26 Jan 2016 19:44:10 +0000 Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h Message-Id: <20160126194410.GQ4503@linux.vnet.ibm.com> List-Id: References: <20160113104516.GE25458@arm.com> <56969F4B.7070001@imgtec.com> <20160113204844.GV6357@twins.programming.kicks-ass.net> <5696BA6E.4070508@imgtec.com> <20160114120445.GB15828@arm.com> <56980145.5030901@imgtec.com> <20160114204827.GE3818@linux.vnet.ibm.com> <56981212.7050301@imgtec.com> <20160114222046.GH3818@linux.vnet.ibm.com> <20160126102402.GE6357@twins.programming.kicks-ass.net> In-Reply-To: <20160126102402.GE6357@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Zijlstra Cc: Leonid Yegoshin , Will Deacon , "Michael S. Tsirkin" , linux-kernel@vger.kernel.org, Arnd Bergmann , linux-arch@vger.kernel.org, Andrew Cooper , Russell King - ARM Linux , virtualization@lists.linux-foundation.org, Stefano Stabellini , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Joe Perches , David Miller , linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-metag@vger.kernel.org, linux-mips@linux-mips.org, x86@kernel.org, user-mode-linux-devel@lists.sourceforge.net, adi-buildroot-devel@lists.sourceforge.ne On Tue, Jan 26, 2016 at 11:24:02AM +0100, Peter Zijlstra wrote: > On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote: > > On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote: > > > On 01/14/2016 12:48 PM, Paul E. McKenney wrote: > > > > > > > >So SYNC_RMB is intended to implement smp_rmb(), correct? > > > Yes. > > > > > > > >You could use SYNC_ACQUIRE() to implement read_barrier_depends() and > > > >smp_read_barrier_depends(), but SYNC_RMB probably does not suffice. > > > > > > If smp_read_barrier_depends() is used to separate not only two reads > > > but read pointer and WRITE basing on that pointer (example below) - > > > yes. I just doesn't see any example of this in famous > > > Documentation/memory-barriers.txt and had no chance to know what you > > > use it in this way too. > > > > Well, Documentation/memory-barriers.txt was intended as a guide for Linux > > kernel hackers, and not for hardware architects. > > Yeah, this goes under the header: memory-barriers.txt is _NOT_ a > specification (I seem to keep repeating this). > > > ------------------------------------------------------------------------ > > > > commit 955720966e216b00613fcf60188d507c103f0e80 > > Author: Paul E. McKenney > > Date: Thu Jan 14 14:17:04 2016 -0800 > > > > documentation: Subsequent writes ordered by rcu_dereference() > > > > The current memory-barriers.txt does not address the possibility of > > a write to a dereferenced pointer. This should be rare, > > How are these rare? Isn't: > > rcu_read_lock() > obj = rcu_dereference(ptr); > if (!atomic_inc_not_zero(&obj->ref)) > obj = NULL; > rcu_read_unlock(); > > a _very_ common thing to do? It is, but it provides its own barriers, so does not need to rely on dependency ordering. Thanx, Paul