Date:	Fri, 26 Jun 2009 17:15:09 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.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

On Fri, Jun 26, 2009 at 10:08:42AM -0400, Gregory Haskins wrote:
> Michael S. Tsirkin wrote:
> > On Fri, Jun 26, 2009 at 09:52:53AM -0400, Gregory Haskins wrote:
> >   
> >> 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
> >>     
> >
> > I think you need v4 otherwise bisect will be broken.
> >   
> 
> I have no problem with a v4, and lets plan on that.  However, note
> bisectability wouldnt be an issue.

Oh, I missed the fact that module_get/module_put check
the module parameter. I take it back then, sorry.

>  GCC would just assign .owner = NULL
> if the client in question doesn't do it explicitly.  All this means is
> that the clients in question would still be broken even if this patch
> went in, but they would be no worse than they are today.
> 
> Note I am technically taking today off, so any respins will probably not
> come out until next week.
> 
> Thanks guys,
> -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;
> >>>>>  }
> >>>>>
> >>>>> --
> >>>>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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> >>>>>   
> >>>>>       
> >>>>>           
> >>>>     
> >>>>         
> >>>   
> >>>       
> >>     
> >
> >
> >   
> 
> 


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