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

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Date:	Fri, 06 Apr 2007 23:40:52 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Hugh Dickins <hugh@...itas.com>,
	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)

Peter Zijlstra wrote:
> On Fri, 2007-04-06 at 23:15 +1000, Nick Piggin wrote:

>>>>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?
> 
> 
> Your lockless pagecache work would solve most of that, no?

Yeah it would.

>>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).
> 
> 
> one could do something like this:
> 
>  struct address_space *mapping = page_mapping(page);
>  if (!mapping || mapping == &swapper_space)
>    do_private_futex();
>  else
>    do_shared_futex(mapping->host, page->index, addr % PAGE_SIZE);
> 
> 
> But alas, it seems I overlooked that the mmap_sem also protects the page
> tables as pointed out by Hugh, so this is all in fain it seems.
> 
> A well.

Well if it turned out to be a real problem, we _could_ introduce a new
futex API that takes a file descriptor+offset.

The old futex API is basically just a shorthand for choosing between
shared and private, depending on what the vma at the given address
happens to be.

-- 
SUSE Labs, Novell Inc.
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