linux-kernel - Re: [RFC PATCH 03/15] Provide atomic_t functions implemented with ISO-C++11 atomics

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Message-ID: <20160523183926.GG3825@linux.vnet.ibm.com>
Date:	Mon, 23 May 2016 11:39:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	linux-arch@...r.kernel.org, x86@...nel.org, will.deacon@....com,
	linux-kernel@...r.kernel.org, ramana.radhakrishnan@....com,
	dwmw2@...radead.org
Subject: Re: [RFC PATCH 03/15] Provide atomic_t functions implemented with
 ISO-C++11 atomics

On Fri, May 20, 2016 at 07:32:09PM +1000, Michael Ellerman wrote:
> On Thu, 2016-05-19 at 08:00 -0700, Paul E. McKenney wrote:
> > On Thu, May 19, 2016 at 04:41:17PM +0200, Peter Zijlstra wrote:
> > > On Thu, May 19, 2016 at 07:22:52AM -0700, Paul E. McKenney wrote:
> > > > Agreed, these sorts of instruction sequences make a lot of sense.
> > > > Of course, if you stuff too many intructions and cache misses between
> > > > the LL and the SC, the SC success probability starts dropping, but short
> > > > seqeunces of non-memory-reference instructions like the above should be
> > > > just fine.
> > > 
> > > In fact, pretty much every single LL/SC arch I've looked at doesn't
> > > allow _any_ loads or stores inside and will guarantee SC failure (or
> > > worse) if you do.
> > 
> > Last I know, PowerPC allowed memory-reference instructions inside, but
> > the more of them you have, the less likely your reservation is to survive.
> > But perhaps I missed some fine print somewhere.  And in any case,
> > omitting them is certainly better.
> 
> There's nothing in the architecture AFAIK.
> 
> Also I don't see anything to indicate that doing more unrelated accesses makes
> the reservation more likely to be lost. Other than it causes you to hold the
> reservation for longer, which increases the chance of some other CPU accessing
> the variable.

And also more likely to hit cache-geometry limitations.

> Having said that, the architecture is written to provide maximum wiggle room
> for implementations. So the list of things that may cause the reservation to be
> lost includes "Implementation-specific characteristics of the coherence
> mechanism", ie. anything.
> 
> > > This immediately disqualifies things like calls/traps/etc.. because
> > > those implicitly issue stores.
> > 
> > Traps for sure.  Not so sure about calls on PowerPC.
> 
> Actually no, exceptions (aka interrupts/traps) are explicitly defined to *not*
> clear the reservation. And function calls are just branches so should also be
> fine.

But don't most interrupt/trap handlers clear the reservation in software?

								Thanx, Paul