linux-kernel - Re: [BUG] staging: android: ashmem: Deadlock during ashmem

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Message-ID: <CAG6tG3xc3RHD7sZXaEbg1vLRhFj+6d1wY=us4BfxBfi5y0o4zg@mail.gmail.com>
Date:	Mon, 22 Apr 2013 10:43:50 -0400
From:	Robert Love <rlove@...gle.com>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Shankar Brahadeeswaran <shankoo77@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bjorn Bringert <bringert@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>, devel@...verdev.osuosl.org,
	Hugh Dickins <hughd@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	anjanavk12@...il.com
Subject: Re: [BUG] staging: android: ashmem: Deadlock during ashmem_shrink

On Mon, Apr 22, 2013 at 10:22 AM, Dan Carpenter
<dan.carpenter@...cle.com> wrote:
> Read Al's email again:  https://lkml.org/lkml/2013/3/20/458
>
> I don't know much about VFS locking, but the ashmem locking seems
> pretty bogus to me.  Why can't multiple threads read() at the same
> time?

ashmem originally did not support read or write operations, just mmap,
which is all 99% of users want. The original concurrency model with
per-mapping ashmem_mutex's works fine there. It is only with the later
addition of read and write that locking becomes a cluster. If there
isn't an obvious way to refactor the locking, I'd suggest removing
read and write.

       Robert
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