Date:	Fri, 26 Jun 2009 16:28:19 +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 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.

> > -------------------------
> >
> > 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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