linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Thu, 7 Jan 2010 12:06:48 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()



On Thu, 7 Jan 2010, Paul E. McKenney wrote:
> > +
> > +	spin_lock(&mm->page_table_lock);
> > +	if (vma->vm_end == cur_brk) {
> > +		vma->vm_end = brk;
> > +		mm->brk = brk;
> > +		cur_brk = brk;
> > +	}
> > +	spin_unlock(&mm->page_table_lock);
> > +
> > +	if (cur_brk != brk)
> 
> Can this be "if (cur_brk < brk)"?  Seems like it should, given the
> earlier tests, but I don't claim to understand the VM code.

It's really just a flag, to test whether the final check (inside the 
spinlock) succeeded, or whether we perhaps raced with _another_ brk() call 
that also had the mm_sem for reading.

We know that cur_brk was different from brk before - because otherwise 
we'd have just returned early (or done the slow case). So testing whether 
it's different afterwards really only tests whether that 

	cur_brk = brk;

statment was executed or not.

I could have used a separate flag called "success" or something. 

		Linus
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