Messages in this thread | | | Date | Mon, 29 Jun 2009 15:30:08 -0400 | From | Gregory Haskins <> | Subject | Re: [PATCH v4] slow-work: add (module*)work->ops->owner to fix races with module clients |
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Gregory Haskins wrote: > (Applies to Linus' linux-2.6.git/master:626f380d) > > [ Changelog: > > v4: > *) added ".owner = THIS_MODULE" fields to all current slow-work > clients (fscache, gfs2). > > 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've retained Michael's "Reviewed-by:" tag from v3 since v4 is identical > in every way except the new hunks added to gfs2/fscache that he asked for. > > Michael, if you want to recind your tag, please speak up. >
Or rescind it even. What a dope I am. <sighs>
-Greg > Otherwise, please consider for inclusion. > > Regards, > -Greg > > --------------------------------- > > slow-work: add (module*)work->ops->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> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com> > CC: David Howells <dhowells@redhat.com> > CC: Steven Whitehouse <swhiteho@redhat.com> > --- > > Documentation/slow-work.txt | 6 +++++- > fs/fscache/object.c | 1 + > fs/fscache/operation.c | 1 + > fs/gfs2/recovery.c | 1 + > include/linux/slow-work.h | 3 +++ > kernel/slow-work.c | 20 +++++++++++++++++++- > 6 files changed, 30 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/fs/fscache/object.c b/fs/fscache/object.c > index 392a41b..d236eb1 100644 > --- a/fs/fscache/object.c > +++ b/fs/fscache/object.c > @@ -45,6 +45,7 @@ static void fscache_enqueue_dependents(struct fscache_object *); > static void fscache_dequeue_object(struct fscache_object *); > > const struct slow_work_ops fscache_object_slow_work_ops = { > + .owner = THIS_MODULE, > .get_ref = fscache_object_slow_work_get_ref, > .put_ref = fscache_object_slow_work_put_ref, > .execute = fscache_object_slow_work_execute, > diff --git a/fs/fscache/operation.c b/fs/fscache/operation.c > index e7f8d53..f1a2857 100644 > --- a/fs/fscache/operation.c > +++ b/fs/fscache/operation.c > @@ -453,6 +453,7 @@ static void fscache_op_execute(struct slow_work *work) > } > > const struct slow_work_ops fscache_op_slow_work_ops = { > + .owner = THIS_MODULE, > .get_ref = fscache_op_get_ref, > .put_ref = fscache_op_put_ref, > .execute = fscache_op_execute, > diff --git a/fs/gfs2/recovery.c b/fs/gfs2/recovery.c > index 59d2695..0c2a6aa 100644 > --- a/fs/gfs2/recovery.c > +++ b/fs/gfs2/recovery.c > @@ -593,6 +593,7 @@ fail: > } > > struct slow_work_ops gfs2_recover_ops = { > + .owner = THIS_MODULE, > .get_ref = gfs2_recover_get_ref, > .put_ref = gfs2_recover_put_ref, > .execute = gfs2_recover_work, > 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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