linux-kernel - Re: [PATCH 1/4] export __put_task

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Message-ID: <20070827181544.GH21089@ftp.linux.org.uk>
Date:	Mon, 27 Aug 2007 19:15:44 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Dean Nelson <dcn@....com>
Cc:	akpm@...ux-foundation.org, linux-ia64@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	tony.luck@...el.com, jes@....com
Subject: Re: [PATCH 1/4] export __put_task_struct for XPMEM

On Mon, Aug 27, 2007 at 01:10:56PM -0500, Dean Nelson wrote:
> On Mon, Aug 27, 2007 at 05:13:28PM +0100, Al Viro wrote:
> > On Mon, Aug 27, 2007 at 10:59:33AM -0500, Dean Nelson wrote:
> > > This patch exports __put_task_struct as it is needed by XPMEM.
> > > 
> > > Signed-off-by: Dean Nelson <dcn@....com>
> > > 
> > > ---
> > > 
> > > One struct file_operations registered by XPMEM, xpmem_open(), calls
> > > 'get_task_struct(current->group_leader)' and another, xpmem_flush(), calls
> > > 'put_task_struct(tg->group_leader)'.
> > 
> > Does it?  Well, then open the file in question and start doing close(dup(fd))
> > in a loop.  Won't take long for an oops...
> 
> Actually it won't oops. And that's because when the file is opened,
> xpmem_open() creates a structure for that thread group, and when
> xpmem_flush() is called on the close() it first looks for that structure
> and if it finds it then it does what it needs to do (which includes the
> put_task_struct() call) and then finishes off by destroying the structure.
> So for subsequent closes xpmem_flush() returns without calling
> put_task_struct().

Then what kind of protection does it get you?  It can be called immediately
after the call of ->open(), so you can't rely on it being there for any
operations.  Makes no sense...
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