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

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Message-ID: <20160519150027.GT3528@linux.vnet.ibm.com>
Date:	Thu, 19 May 2016 08:00:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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 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.

> This immediately disqualifies things like calls/traps/etc.. because
> those implicitly issue stores.

Traps for sure.  Not so sure about calls on PowerPC.

							Thanx, Paul