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

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Date:	Wed, 26 Feb 2014 10:27:47 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Torvald Riegel <triegel@...hat.com>
Cc:	Michael Matz <matz@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Wed, Feb 26, 2014 at 02:04:30PM +0100, Torvald Riegel wrote:
> xagsmtp2.20140226130517.3625@...dvma.vnet.ibm.com
> X-Xagent-Gateway: vmsdvma.vnet.ibm.com (XAGSMTP2 at VMSDVMA)
> 
> On Fri, 2014-02-21 at 11:13 -0800, Paul E. McKenney wrote:
> > On Fri, Feb 21, 2014 at 07:35:37PM +0100, Michael Matz wrote:
> > > Hi,
> > > 
> > > On Thu, 20 Feb 2014, Linus Torvalds wrote:
> > > 
> > > > But I'm pretty sure that any compiler guy must *hate* that current odd
> > > > dependency-generation part, and if I was a gcc person, seeing that
> > > > bugzilla entry Torvald pointed at, I would personally want to
> > > > dismember somebody with a rusty spoon..
> > > 
> > > Yes.  Defining dependency chains in the way the standard currently seems 
> > > to do must come from people not writing compilers.  There's simply no 
> > > sensible way to implement it without being really conservative, because 
> > > the depchains can contain arbitrary constructs including stores, 
> > > loads and function calls but must still be observed.  
> > > 
> > > And with conservative I mean "everything is a source of a dependency, and 
> > > hence can't be removed, reordered or otherwise fiddled with", and that 
> > > includes code sequences where no atomic objects are anywhere in sight [1].
> > > In the light of that the only realistic way (meaning to not have to 
> > > disable optimization everywhere) to implement consume as currently 
> > > specified is to map it to acquire.  At which point it becomes pointless.
> > 
> > No, only memory_order_consume loads and [[carries_dependency]]
> > function arguments are sources of dependency chains.
> 
> However, that is, given how the standard specifies things, just one of
> the possible ways for how an implementation can handle this.  Treating
> [[carries_dependency]] as a "necessary" annotation to make exploiting
> mo_consume work in practice is possible, but it's not required by the
> standard.
> 
> Also, dependencies are specified to flow through loads and stores
> (restricted to scalar objects and bitfields), so any load that might
> load from a dependency-carrying store can also be a source (and that
> doesn't seem to be restricted by [[carries_dependency]]).

OK, this last is clearly totally unacceptable.  :-/

Leaving aside the option of dropping the whole thing for the moment,
the only thing that suggests itself is having all dependencies die at
a specific point in the code, corresponding to the rcu_read_unlock().
But as far as I can see, that absolutely requires "necessary" parameter
and return marking in order to correctly handle nested RCU read-side
critical sections in different functions.

							Thanx, Paul

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