linux-kernel - Re: [PATCH 2/2] vfs: fix d_path() for unreachable paths

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Date:	Mon, 21 Sep 2009 15:38:57 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Valdis.Kletnieks@...edu, agruen@...e.de, hch@....de,
	hugh.dickins@...cali.co.uk, matthew@....cx
Subject: Re: [PATCH 2/2] vfs: fix d_path() for unreachable paths

On Mon, Sep 21, 2009 at 10:10:17AM -0400, Mike Frysinger wrote:

> it works without having to copy & paste the same exact structures over
> and over.  a suggestion as how to do it cleanly without bloating the
> code is certainly welcome.  it doesnt really matter that it's on the
> stack as the usage is small and d_path() is given the size of the
> buffer, so it isnt going to overflow.

Umm...  Surely, you can put a CPU number + one bit into PDE->data?  As in
	proc_create_data("icplb", ....., (void *)(cpu * 2))
	proc_create_data("dcplb", ....., (void *)(cpu * 2 + 1))
and
	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
	unsigned long n = (unsigned long) pde->data;
	...
	cpu = n / 2;
	is_D = n & 1;

> > ??For another... locking in that loop
> > over processes and VMAs of each looks very suspicios, while we are at it.
> 
> we've reviewed it several times and exercised it in multiple ways.  so
> unless you have a real idea of something being wrong, the code has
> been vetted heavily.

write_lock_irq tasklist_lock
loop over processes
	get_task_mm [now VM is pinned down, but not locked]
	walk the VMA list without any locking

Something on another CPU (you have SMP systems, judging by the previously
discussed code, right?) does munmap().

Boom.
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