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

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Date:	Fri, 06 Apr 2007 23:15:33 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Eric Dumazet <dada1@...mosbay.com>,
	Ulrich Drepper <drepper@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>, 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)

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:

I'd imagine shared futexes would be much less common than private for
threaded programs... I'd say we should reevaluate things once we have
private futexes, and malloc/free stop hammering mmap_sem so hard...

>> - get a page using pfn_to_page (skipping VM_PFNMAP)
>> - get the futex key from page->mapping->host and page->index
>>   and offset from addr % PAGE_SIZE.
>>
>>or given a key:
>>
>> - lookup the page from key.shared.inode->i_mapping by key.shared.pgoff
>>   possibly loading the page using mapping->a_ops->readpage().

For shared futexes, wouldn't i_mapping be worse, because you'd be
ping-ponging the tree_lock between processes, rather than have each
use their own mmap_sem?

That also only helps for the wakeup case too, doesn't it? You have
to use the vmas to find out which inode to use to do the wait, I think?
(unless you introduce a new shared futex API).

-- 
SUSE Labs, Novell Inc.
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