linux-kernel - Re: [PATCH, v3] shmat: introduce flag SHM_MAP_NOT

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Message-ID: <20081008110243.GN7971@one.firstfloor.org>
Date:	Wed, 8 Oct 2008 13:02:43 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Andi Kleen <andi@...stfloor.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Hugh Dickins <hugh@...itas.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH, v3] shmat: introduce flag SHM_MAP_NOT_FIXED

On Wed, Oct 08, 2008 at 11:20:37AM +0100, Alan Cox wrote:
> > That is racy when multi threaded because shmat() doesn't replace, so you 
> > would need to munmap() inbetween and someone else could steal the area
> > then. Yes you could stick a loop around it. It could livelock.
> > No, it's not a good interface I would advocate.
> 
> You could just use pthread mutexes in your application. The rôle of the

malloc() can call mmap, so that would require putting a mutex around
each malloc(). Good luck finding them all.

> kernel is not to provide nappies for people who think programming is too
> hard but to provide services that can be used to build applications.

Outsourcing kernel locking to user space is not the way to go.

-Andi

-- 
ak@...ux.intel.com
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