linux-kernel - Re: Is module refcounting racy?

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Message-ID: <e619185d1003290958w23037176uc869c7dbc0a48bcc@mail.gmail.com>
Date:	Tue, 30 Mar 2010 03:58:49 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Is module refcounting racy?

On Mon, Mar 29, 2010 at 8:12 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> On Thu, 18 Mar 2010 09:25:34 pm Nick Piggin wrote:
>> Hey,
>>
>> I've been looking at weird and wonderful ways to do scalable refcounting,
>> for the vfs...
>>
>> Sadly, module refcounting doesn't fit my bill. But as far as I could see,
>> it is racy.
>
> Other than for advisory purposes, the refcount is only checked against zero
> under stop_machine.  For exactly this reason.

There definitely looks to me like there is code that checks the refcount
*without* stop_machine. module_refcount is an exported function, and you
expect drivers to get this right (scsi_device_put for a trivial example), but
it even looks like it is used in a racy way in kernel/module.c code.

Either we need to take my patch, or audit t, and put a WARN_ON
if it is called while not under stop_machine.
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