linux-kernel - Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4 released

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Date:	Thu, 21 Aug 2008 16:48:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	jmerkey@...fmountaingroup.com,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [ANNOUNCE] mdb: Merkey's Linux Kernel Debugger 2.6.27-rc4
	released

On Thu, 2008-08-21 at 07:30 -0700, Paul E. McKenney wrote:
> On Thu, Aug 21, 2008 at 04:09:59PM +0200, Peter Zijlstra wrote:
> > On Thu, 2008-08-21 at 23:37 +1000, Nick Piggin wrote:
> > > On Thursday 21 August 2008 22:26, jmerkey@...fmountaingroup.com wrote:
> > > 
> > > > I used the smp_wmb() functions.  I noted a couple of things.  a) some of
> > > > these macros just emit __asm__ __volatile__ into the code so why not just
> > > > say "volatile" to begin with
> > > 
> > > It is not the same as volatile type. What it does is tell the compiler
> > > to clobber all registers or temporaries. This something pretty well
> > > defined and hard to get wrong compared to volatile type.
> > 
> > Right, asm volatile () means that the asm may not be discarted. Very
> > different from the volatile type qualifier.
> > 
> > > > b) smp_wmb() in some cases worked and in 
> > > > other cases jut optimized away the global reference.
> > > 
> > > Linux barriers aren't going to force a load to be emitted, if it can be
> > > optimized away. If it optimized away a store, then I'd like to see a
> > > test case.
> > 
> > Not sure - I think all barrier clobber the full register and memory set.
> > So if you access a variable after a barrier it will have to issue a
> > load.
> 
> Here is one example (which might or might not be what Nick had in mind):
> 
> 	extern int v;
> 
> 	void foo(void)
> 	{
> 		do_something_with(v);
> 		barrier();
> 		do_something_else_with(v - v);
> 	}
> 
> The second set of loads from v can be optimized away unless v is
> declared volatile.  In contrast:
> 
> 	void bar(void)
> 	{
> 		do_something_with(v);
> 		barrier();
> 		do_something_else_with(v);
> 	}
> 
> Here the compiler must refetch v after the barrier.

Ah, right. But in that case:

 v-v := tmp1 = v; tmp2 = v; sub tmp1,tmp2;

Which you can of course write out more explicitly in C as well and
insert a barrier between the two reads of v, giving the same effect as
volatile.

Still, point taken.

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