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Message-ID: <20151005165703.GD4487@linux.vnet.ibm.com> Date: Mon, 5 Oct 2015 09:57:03 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Will Deacon <will.deacon@....com> Cc: Peter Zijlstra <peterz@...radead.org>, Boqun Feng <boqun.feng@...il.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, Ingo Molnar <mingo@...nel.org>, Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Thomas Gleixner <tglx@...utronix.de>, Waiman Long <waiman.long@...com> Subject: Re: [RFC v2 4/7] powerpc: atomic: Implement xchg_* and atomic{,64}_xchg_* variants On Mon, Oct 05, 2015 at 03:44:07PM +0100, Will Deacon wrote: > On Thu, Oct 01, 2015 at 07:03:01PM +0100, Paul E. McKenney wrote: > > On Thu, Oct 01, 2015 at 07:13:04PM +0200, Peter Zijlstra wrote: > > > On Thu, Oct 01, 2015 at 08:09:09AM -0700, Paul E. McKenney wrote: > > > > On Thu, Oct 01, 2015 at 02:24:40PM +0200, Peter Zijlstra wrote: > > > > > > > > I must say I'm somewhat surprised by this level of relaxation, I had > > > > > expected to only loose SMP barriers, not the program order ones. > > > > > > > > > > Is there a good argument for this? > > > > > > > > Yes, when we say "relaxed", we really mean relaxed. ;-) > > > > > > > > Both the CPU and the compiler are allowed to reorder around relaxed > > > > operations. > > > > > > Is this documented somewhere, because I completely missed this part. > > > > Well, yes, these need to be added to the documentation. I am assuming > > that Will is looking to have the same effect as C11 memory_order_relaxed, > > which is relaxed in this sense. If he has something else in mind, > > he needs to tell us what it is and why. ;-) > > I was treating them purely as being single-copy atomic and not providing > any memory ordering guarantees (much like the non *_return atomic operations > that we already have). I think this lines up with C11, minus the bits > about data races which we don't call out anyway. As long as it is single-copy atomic and not multi-copy atomic, I believe we are on the saem page. We have slowly been outlawing some sorts of data races over the past few years, and I would guess that this will continue, expecially if good tooling emerges (and KTSAN is showing some promise from what I can see). Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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