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 > > 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 > >>> ------------------------- >>> >>> 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 >>> CC: David Howells >>> --- >>> >>> 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 >>> +#include >>> >>> 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/ >>> >>> >> > > >