Messages in this thread | | | Date | Fri, 26 Jun 2009 17:02:19 +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 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@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. > > > > 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.
> > > >>> ------------------------- > >>> > >>> 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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