Date:	Fri, 26 Jun 2009 09:52:53 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	dhowells@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] slow-work: add (module*)work->owner to fix races	with
 module clients

Michael S. Tsirkin wrote:
> On Fri, Jun 26, 2009 at 08:00:45AM -0400, Gregory Haskins wrote:
>   
>> Gregory Haskins wrote:
>>     
>>> (Try 3: applies to Linus' git master:626f380d)
>>>
>>> [ Changelog:
>>>
>>>     v3:
>>>         *) moved (module*)owner to slow_work_ops 
>>>         *) removed useless barrier()
>>> 	*) updated documentation/comments 
>>>
>>>     v2:
>>> 	*) cache "owner" value to prevent invalid access after put_ref
>>>
>>>     v1:
>>> 	*) initial release
>>> ]
>>>
>>>   
>>>       
>> (I know there were several versions of this patch floating around.  This
>> was compounded by the fact that I had also originally submitted it as
>> part of a larger series against KVM and those problems I had with my
>> mailer.  But FWIW: This is the latest version to consider for merging to
>> mainline.  I've CC'd Michael Tsirkin who has reviewed this patch. 
>> Perhaps I can prod an Acked-by/Reviewed-by tag out of him ;) )
>>
>> Kind Regards,
>> -Greg
>>     
>
> The race itself seems to be real, and the patch looks good to me.
> There's ongoing discussion on whether KVM needs to use slow-work,
> but there are other modular users which will benefit from this.
>
> Reviewed-by: Michael S. Tsirkin <mst@...hat.com>
>
> By the way: I think you also need to update all users, which include
> at least GFS2 and fscache, to init the owner field.
>   

Good catch!  That was a side effect of v3 since v2 used to have the
owner in the slow_work and do the init implicitly in slow_work_init(). 
Should I respin a v4 with those new hunks, or should we patch those
separately?

-Greg
>   
>>> -------------------------
>>>
>>> slow-work: add (module*)work->owner to fix races with module clients
>>>
>>> The slow_work facility was designed to use reference counting instead of
>>> barriers for synchronization.  The reference counting mechanism is
>>> implemented as a vtable op (->get_ref, ->put_ref) callback.  This is
>>> problematic for module use of the slow_work facility because it is
>>> impossible to synchronize against the .text installed in the callbacks:
>>> There is no way to ensure that the slow-work threads have completely
>>> exited the .text in question and rmmod may yank it out from under the
>>> slow_work thread.
>>>
>>> This patch attempts to address this issue by mapping "struct module* owner"
>>> to the slow_work_ops item, and maintaining a module reference
>>> count coincident with the more externally visible reference count.  Since
>>> the slow_work facility is resident in kernel, it should be a race-free
>>> location to issue a module_put() call.  This will ensure that modules
>>> can properly cleanup before exiting.
>>>
>>> A module_get()/module_put() pair on slow_work_enqueue() and the subsequent
>>> dequeue technically adds the overhead of the atomic operations for every
>>> work item scheduled.  However, slow_work is designed for deferring
>>> relatively long-running and/or sleepy tasks to begin with, so this
>>> overhead will hopefully be negligible.
>>>
>>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>>> CC: David Howells <dhowells@...hat.com>
>>> ---
>>>
>>>  Documentation/slow-work.txt |    6 +++++-
>>>  include/linux/slow-work.h   |    3 +++
>>>  kernel/slow-work.c          |   20 +++++++++++++++++++-
>>>  3 files changed, 27 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/slow-work.txt b/Documentation/slow-work.txt
>>> index ebc50f8..2a38878 100644
>>> --- a/Documentation/slow-work.txt
>>> +++ b/Documentation/slow-work.txt
>>> @@ -80,6 +80,7 @@ Slow work items may then be set up by:
>>>   (2) Declaring the operations to be used for this item:
>>>  
>>>  	struct slow_work_ops myitem_ops = {
>>> +	        .owner   = THIS_MODULE,
>>>  		.get_ref = myitem_get_ref,
>>>  		.put_ref = myitem_put_ref,
>>>  		.execute = myitem_execute,
>>> @@ -102,7 +103,10 @@ A suitably set up work item can then be enqueued for processing:
>>>  	int ret = slow_work_enqueue(&myitem);
>>>  
>>>  This will return a -ve error if the thread pool is unable to gain a reference
>>> -on the item, 0 otherwise.
>>> +on the item, 0 otherwise.  Loadable modules may only enqueue work if at least
>>> +one reference to the module is known to be held.  The slow-work infrastructure
>>> +will acquire a reference to the module and hold it until after the item's
>>> +reference is dropped, assuring the stability of the callback.
>>>  
>>>  
>>>  The items are reference counted, so there ought to be no need for a flush
>>> diff --git a/include/linux/slow-work.h b/include/linux/slow-work.h
>>> index b65c888..1382918 100644
>>> --- a/include/linux/slow-work.h
>>> +++ b/include/linux/slow-work.h
>>> @@ -17,6 +17,7 @@
>>>  #ifdef CONFIG_SLOW_WORK
>>>  
>>>  #include <linux/sysctl.h>
>>> +#include <linux/module.h>
>>>  
>>>  struct slow_work;
>>>  
>>> @@ -24,6 +25,8 @@ struct slow_work;
>>>   * The operations used to support slow work items
>>>   */
>>>  struct slow_work_ops {
>>> +	struct module *owner;
>>> +
>>>  	/* get a ref on a work item
>>>  	 * - return 0 if successful, -ve if not
>>>  	 */
>>> diff --git a/kernel/slow-work.c b/kernel/slow-work.c
>>> index 09d7519..18dee34 100644
>>> --- a/kernel/slow-work.c
>>> +++ b/kernel/slow-work.c
>>> @@ -145,6 +145,15 @@ static unsigned slow_work_calc_vsmax(void)
>>>  	return min(vsmax, slow_work_max_threads - 1);
>>>  }
>>>  
>>> +static void slow_work_put(struct slow_work *work)
>>> +{
>>> +	/* cache values that are needed during/after pointer invalidation */
>>> +	struct module *owner = work->ops->owner;
>>> +
>>> +	work->ops->put_ref(work);
>>> +	module_put(owner);
>>> +}
>>> +
>>>  /*
>>>   * Attempt to execute stuff queued on a slow thread.  Return true if we managed
>>>   * it, false if there was nothing to do.
>>> @@ -219,7 +228,7 @@ static bool slow_work_execute(void)
>>>  		spin_unlock_irq(&slow_work_queue_lock);
>>>  	}
>>>  
>>> -	work->ops->put_ref(work);
>>> +	slow_work_put(work);
>>>  	return true;
>>>  
>>>  auto_requeue:
>>> @@ -299,6 +308,14 @@ int slow_work_enqueue(struct slow_work *work)
>>>  		if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
>>>  			set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
>>>  		} else {
>>> +			/*
>>> +			 * Callers must ensure that their module has at least
>>> +			 * one reference held while the work is enqueued.  We
>>> +			 * will acquire another reference here and drop it
>>> +			 * once we do the last ops->put_ref()
>>> +			 */
>>> +			__module_get(work->ops->owner);
>>> +
>>>  			if (work->ops->get_ref(work) < 0)
>>>  				goto cant_get_ref;
>>>  			if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
>>> @@ -313,6 +330,7 @@ int slow_work_enqueue(struct slow_work *work)
>>>  	return 0;
>>>  
>>>  cant_get_ref:
>>> +	module_put(work->ops->owner);
>>>  	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
>>>  	return -EAGAIN;
>>>  }
>>>
>>> --
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>>>   
>>>       
>>     
>
>
>   



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