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Date: Tue, 26 Jan 2016 11:44:10 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>, Will Deacon <will.deacon@....com>, "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org, Andrew Cooper <andrew.cooper3@...rix.com>, Russell King - ARM Linux <linux@....linux.org.uk>, virtualization@...ts.linux-foundation.org, Stefano Stabellini <stefano.stabellini@...citrix.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>, Joe Perches <joe@...ches.com>, David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org, sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-metag@...r.kernel.org, linux-mips@...ux-mips.org, x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net, adi-buildroot-devel@...ts.sourceforge.net, linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org, xen-devel@...ts.xenproject.org, Ralf Baechle <ralf@...ux-mips.org>, Ingo Molnar <mingo@...nel.org>, ddaney.cavm@...il.com, james.hogan@...tec.com, Michael Ellerman <mpe@...erman.id.au> Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h 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 <paulmck@...ux.vnet.ibm.com> > > 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
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