linux-kernel - Re: [PATCH] Simplify /proc/<pid|self>/exe symlink code

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Date:	Fri, 13 Jul 2007 12:54:20 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Al Viro <viro@....linux.org.uk>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	"Hallyn, Serge" <serue@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] Simplify /proc/<pid|self>/exe symlink code

On Thu, 2007-07-12 at 19:21 -0700, Andrew Morton wrote:
> On Thu, 12 Jul 2007 19:00:12 -0700 Matt Helsley <matthltc@...ibm.com> wrote:
> 
> > This patch avoids holding the mmap semaphore while walking VMAs in response to
> > programs which read or follow the /proc/<pid|self>/exe symlink. This also allows
> > us to merge mmu and nommu proc_exe_link() functions. The costs are holding the
> > task lock, a separate reference to the executable file stored in the task
> > struct, and increased code in fork, exec, and exit paths.
> > 
> > Signed-off-by: Matt Helsley <matthltc@...ibm.com>
> > ---
> > 
> > Changelog:
> > 
> > Hold task_lock() while using task->exe_file. With this change I haven't
> > 	been able to reproduce Chris Wright's Oops report:
> > 		http://lkml.org/lkml/2007/5/31/34 
> > 	I used a 4-way, x86 system running kernbench. I also tried a 4-way x86_64
> > 	system running pidof. I used oprofile during all runs but I could not
> > 	reproduce Chris' Oops with the new patch.
> > 
> > Compiled and passed simple tests for regressions when patched against a 2.6.20
> > and a 2.6.22 kernel. Regression tests included a variety of file operations on
> > /proc/<pid|self>/exe such as stat, lstat, open, close, readlink, and unlink. All
> > produced the expected, baseline output results.
> > 
> > Andrew, please consider this patch for inclusion in -mm.
> 
> I wish we had a description of the bug which this fixes.  That email of
> Chris's is referencing code which diddles with task_struct.exe_file, but
> your patch _adds_ task_struct.exe_file, so I am all confused.

Chris was testing the patch. The patch isn't a bug fix so much as a
failed attempt to remove one use of the mm's mmap lock.

> Your patch does lots of fput()s under task_lock(), but fput() can sleep.

Ack, you're right.

> Plus what Al said.

Yup.

Thanks,
	-Matt Helsley

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