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Date: Fri, 06 Apr 2007 23:15:33 +1000
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Hugh Dickins <hugh@veritas.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Eric Dumazet <dada1@cosmosbay.com>, Ulrich Drepper <drepper@gmail.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Dave Jones <davej@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Andi Kleen <ak@suse.de>, Ravikiran G Thirumalai <kiran@scalex86.org>,
       "Shai Fultheim (Shai@scalex86.org)" <shai@scalex86.org>,
       pravin b shelar <pravin.shelar@calsoftinc.com>,
       linux-kernel@vger.kernel.org,
       "Pierre.Peiffer" <Pierre.Peiffer@bull.net>
Subject: Re: Shared futexes (was [PATCH] FUTEX : new PRIVATE futexes)
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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.