linux-kernel - Re: [PATCH] module: Fix race condition between load and unload module

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Date:	Sat, 13 Apr 2013 14:10:11 -0700
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Salman Qazi <sqazi@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] module: Fix race condition between load and unload module

Hi

On Sat, Apr 13, 2013 at 10:53 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Sat, Apr 13, 2013 at 8:41 AM, Anatol Pomozov
> <anatol.pomozov@...il.com> wrote:
>>
>> Does it make sense to move it to a separate function in kref.h?
>>
>> /** Useful when kref_get is racing with kref_put and refcounter might be 0 */
>> int kref_get_not_zero(kref* ref) {
>>     return atomic_inc_not_zero(&kref->refcount);
>> }
>
> It turns out we have that, except it's called "unless_zero", because
> it uses "atomic_add_unless(x,1,0)", rather than the simplified
> "atomic_inc_not_zero(x)".
>
>> or maybe instead change default behavior of kref_get() to
>> atomic_inc_not_zero and force callers check the return value from
>> kref_get()?
>
> That would be painful, and _most_ users should have a preexisting
> refcount. So it's probably better in the long run to just keep the
> warning (but perhaps fix it to be SMP-safe). So I think the part of
> your patch that made kref_get() use atomic_inc_return() is probably a
> good idea regardless.
>
> Also, I changed my patch to be minimal, and not change other users of
> kobject_get(). So other users (not kset_find_obj()) will continue to
> get the warning, and kset_find_obj() uses the safe version.
Looks good to me.

> So this is
> what I'm planning on committing as the minimal patch and marking for
> stable. The rest (including that atomic_inc_return() in kref_get)
> would be cleanup.
>
> Can you give this a quick test?

I ran the test case for ~60 minutes with XFS tests in parallel - no any issues.
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