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SubjectRe: [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@redhat.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@novell.com>
> > CC: David Howells <dhowells@redhat.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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