linux-kernel - Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS

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Date:	Thu, 16 Apr 2015 12:41:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	Jason Low <jason.low2@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	hideaki.kimura@...com, Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the
 scheduler

On Thu, Apr 16, 2015 at 09:02:08PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 16, 2015 at 08:24:27PM +0200, Ingo Molnar wrote:
> > Yes ... but that still leaves this weird feeling that it's really 
> > still a bit wrong because it's not proper parallel code, we just 
> > reduced the probability of the remaining races radically. And it's not 
> > like GCC (or any compiler) does load tearing or even store tearing 
> > under normal -O2 for such code patterns, right?
> 
> I think Paul once caught GCC doing something silly, but typically no.
> The re-loads however have been frequently observed.

Too true!

Some architectures do split stores of constants.  For example, given
an architecture with a store-immediate instruction with (say) a four-bit 
immediate field, gcc can compile this:

	x = 0x00020008;

to something like:

	st $2, (x+2)
	st $8, (x)

And gcc was doing this even though the store to x had volatile semantics,
a bug which has thankfully since been fixed.

But then again, I am paranoid.  So I would not put it past gcc to think
to itself "Hmmm...  I just loaded x a few instructions back, and only
clobbered the low-order byte.  So I will just reload that byte into
low-order byte of the register containing the remnants of the previous
load."

No, I have never seen gcc do that, but a C compiler could do that and
still claim to be complying with the standard.  :-/

							Thanx, Paul

> > > And its not like they really cost anything.
> > 
> > That's true.
> > 
> > Would it make sense to add a few comments to the seq field definition 
> > site(s), about how it's supposed to be accessed - or to the 
> > READ_ONCE()/WRITE_ONCE() sites, to keep people from wondering?
> 
> For sure, can do a comment no problem.
> 

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