linux-kernel - Re: [RFC][PATCH 0/5] arch: atomic rework

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Message-ID: <20140221223741.GO4250@linux.vnet.ibm.com>
Date:	Fri, 21 Feb 2014 14:37:41 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Joseph S. Myers" <joseph@...esourcery.com>
Cc:	Michael Matz <matz@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Torvald Riegel <triegel@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Fri, Feb 21, 2014 at 10:10:54PM +0000, Joseph S. Myers wrote:
> On Fri, 21 Feb 2014, Paul E. McKenney wrote:
> 
> > This needs to be as follows:
> > 
> > [[carries_dependency]] int getzero(int i [[carries_dependency]])
> > {
> > 	return i - i;
> > }
> > 
> > Otherwise dependencies won't get carried through it.
> 
> C11 doesn't have attributes at all (and no specification regarding calls 
> and dependencies that I can see).  And the way I read the C++11 
> specification of carries_dependency is that specifying carries_dependency 
> is purely about increasing optimization of the caller: that if it isn't 
> specified, then the caller doesn't know what dependencies might be 
> carried.  "Note: The carries_dependency attribute does not change the 
> meaning of the program, but may result in generation of more efficient 
> code. - end note".

Good point -- I am so used to them being in gcc that I missed that.

In which case, it seems to me that straight C11 is within its rights
to emit a memory barrier just before control passes into a function
that either it can't see or that it chose to apply dependency-breaking
optimizations to.

							Thanx, Paul

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