linux-kernel - Re: Shared futexes (was [PATCH] FUTEX : new PRIVATE futexes)

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Date:	Fri, 06 Apr 2007 15:15:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Ulrich Drepper <drepper@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	"Shai Fultheim (Shai@...lex86.org)" <shai@...lex86.org>,
	pravin b shelar <pravin.shelar@...softinc.com>,
	linux-kernel@...r.kernel.org,
	"Pierre.Peiffer" <Pierre.Peiffer@...l.net>
Subject: Re: Shared futexes (was [PATCH] FUTEX : new PRIVATE futexes)

On Fri, 2007-04-06 at 14:02 +0100, Hugh Dickins wrote:
> On Fri, 6 Apr 2007, Peter Zijlstra wrote:
> > 
> > some thoughts on shared futexes;
> > 
> > Could we get rid of the mmap_sem on the shared futexes in the following
> > manner:
> > 
> >  - do a page table walk to find the pte;
> 
> ("walk" meaning descent down the levels, I presume, rather than across)

indeed.

> I've not had time to digest your proposal, and I'm about to go out:
> let me sound a warning that springs to mind, maybe it's entirely
> inapproriate, but better said than kept silent.
> 
> It looks as if you're supposing that mmap_sem is needed to find_vma,
> but not for going down the pagetables.  It's not a simple as that:
> you need to be careful that a concurrent munmap from another thread
> isn't freeing pagetables from under you.
> 
> Holding (down_read) of mmap_sem is one way to protect against that.
> try_to_unmap doesn't have that luxury: in its case, it's made safe
> by the way free_pgtables does anon_vma_unlink and unlink_file_vma
> before freeing any pagetables, so try_to_unmap etc. won't get there;
> but you can't do that.

Ah, drad.

I had hoped that the pte_lock would solve that race, but that doesn't
cover the upper levels of the tree.

Back to the drawing board..

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