From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Brice Goglin <Brice.Goglin@ens-lyon.org>
Cc: Matt Mackall <mpm@selenic.com>,
Alexey Dobriyan <adobriyan@gmail.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Gerald Schaefer <gerald.schaefer@de.ibm.com>,
akpm@linux-foundation.org
Subject: Re: [PATCH] acquire mmap semaphore in pagemap_read.
Date: Thu, 12 Mar 2009 16:46:34 +0100 [thread overview]
Message-ID: <20090312164634.6f2027ac@skybase> (raw)
In-Reply-To: <49B92D2B.2090100@ens-lyon.org>
On Thu, 12 Mar 2009 16:41:31 +0100
Brice Goglin <Brice.Goglin@ens-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.
WARNING: multiple messages have this Message-ID (diff)
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Brice Goglin <Brice.Goglin@ens-lyon.org>
Cc: Matt Mackall <mpm@selenic.com>,
Alexey Dobriyan <adobriyan@gmail.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Gerald Schaefer <gerald.schaefer@de.ibm.com>,
akpm@linux-foundation.org
Subject: Re: [PATCH] acquire mmap semaphore in pagemap_read.
Date: Thu, 12 Mar 2009 16:46:34 +0100 [thread overview]
Message-ID: <20090312164634.6f2027ac@skybase> (raw)
In-Reply-To: <49B92D2B.2090100@ens-lyon.org>
On Thu, 12 Mar 2009 16:41:31 +0100
Brice Goglin <Brice.Goglin@ens-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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next prev parent reply other threads:[~2009-03-12 15:50 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-03-12 10:33 [PATCH] acquire mmap semaphore in pagemap_read Martin Schwidefsky
2009-03-12 10:33 ` Martin Schwidefsky
2009-03-12 11:45 ` Alexey Dobriyan
2009-03-12 11:45 ` Alexey Dobriyan
2009-03-12 11:54 ` Martin Schwidefsky
2009-03-12 11:54 ` Martin Schwidefsky
2009-03-12 15:23 ` Matt Mackall
2009-03-12 15:23 ` Matt Mackall
2009-03-12 15:27 ` Martin Schwidefsky
2009-03-12 15:27 ` Martin Schwidefsky
2009-03-12 15:41 ` Brice Goglin
2009-03-12 15:41 ` Brice Goglin
2009-03-12 15:46 ` Martin Schwidefsky [this message]
2009-03-12 15:46 ` Martin Schwidefsky
2009-03-12 15:54 ` Martin Schwidefsky
2009-03-12 15:54 ` Martin Schwidefsky
2009-03-17 12:04 ` Martin Schwidefsky
2009-03-17 12:04 ` Martin Schwidefsky
2009-03-17 16:21 ` Matt Mackall
2009-03-17 16:21 ` Matt Mackall
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