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

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Message-Id: <1175862369.6483.173.camel@twins>
Date:	Fri, 06 Apr 2007 14:26:09 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	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, hugh <hugh@...itas.com>,
	"Pierre.Peiffer" <Pierre.Peiffer@...l.net>
Subject: Shared futexes (was [PATCH] FUTEX : new PRIVATE futexes)


Hi,

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;
 - 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().

then:

 - perform the futex operation on a kmap of the page


This should all work except for VM_PFNMAP.

Since the address is passed from userspace we cannot trust it to not
point into a VM_PFNMAP area.

However, with the RCU VMA lookup patches I'm working on we could do that
check without holding locks and without exclusive cachelines; the
question is, is that good enough?

Or is there an alternative way of determining a pfnmap given a
pfn/struct page?

-
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