Messages in this thread | | | Date | Fri, 26 Jun 2009 16:28:19 +0300 | From | "Michael S. Tsirkin" <> | Subject | Re: [PATCH v3] slow-work: add (module*)work->owner to fix races with module clients |
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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; > > } > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > > > >
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