linux-kernel - Re: Race condition: calling remove_proc_entry in cleanup_module (module

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Message-ID: <379fb4870709040945ve834f9djf4772ec6166bb7ff@mail.gmail.com>
Date:	Tue, 4 Sep 2007 18:45:23 +0200
From:	"anon... anon.al" <anon.asdf@...il.com>
To:	albert.neu@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Race condition: calling remove_proc_entry in cleanup_module (module_exit) while someone's using procfile

On 9/4/07, anon... anon.al <anon.asdf@...il.com> wrote:
<snip>
> If yes: which mechanism can be used?

I was thinking about using an atomic counter in procfile_write

  proc_f = create_proc_entry(PROC_FILE_NAME, 0644, NULL);
  //...
  proc_f->write_proc = procfile_write;

int procfile_write(struct file *filp, const char *buffer, \
		   unsigned long len, void *data)
{
  //"StackXXX"
  atomic_inc(&cnt_procfile_users);

  printk(KERN_ALERT "Hi there!\n");

  atomic_dec(&cnt_procfile_users);
  wake_up_interruptible(&queue);
  return len;
}

and then in cleanup_module using:

wait_event_interruptible(queue,           \
(                                         \
 spin_lock_irqsave(&lock, flags),	  \
 cnt = atomic_read(&cnt_procfile_users),  \
 ((cnt == 0)                              \
  ? 1					  \
  : (spin_unlock_irqrestore(&lock, flags), 0))\
));
remove_proc_entry(PROC_FILE_NAME, &proc_root);
spin_unlock_irqrestore(&lock, flags);

But:
x1)
Could it happen that code is already in function procfile_write at "StackXXX"
(before atomic_inc(&cnt_procfile_users)) when the scheduler switches
to another task??
((Or is the "entering into a function, up to the function's first
statement" atomic??))

x2)
Could it happen that the scheduler switches, after
atomic_dev(&cnt_procfile_users) but before
return len??

If so, then it could happen that we're in spin_lock_irqsave, while
someone else is still using the procfile; and then this code still
fails miserably.
?

Regards -Albert
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