linux-kernel - Re: [PATCH] acquire mmap semaphore in pagemap

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Date:	Thu, 12 Mar 2009 16:46:34 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Brice Goglin <Brice.Goglin@...-lyon.org>
Cc:	Matt Mackall <mpm@...enic.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Gerald Schaefer <gerald.schaefer@...ibm.com>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] acquire mmap semaphore in pagemap_read.

On Thu, 12 Mar 2009 16:41:31 +0100
Brice Goglin <Brice.Goglin@...-lyon.org> wrote:

> > Which would be really ugly. I still have not grasped why this will
> > introduce a deadlock though. The worst the put_user can do is to cause
> > a page fault, no? I do not see where the fault handler acquires the
> > mmap_sem as writer. It takes the mmap_sem as reader and two readers
> > should be fine.
> 
> Somebody else can acquire for write in the meantime, for instance
> another thread doing mprotect. This writer is blocked by the first
> reader, and the second reader is blocked by the writer. So both
> tasks are blocked.

I see, fair r/w locks. So nested down_read is a no-no.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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