Messages in this thread Patch in this message | | | From | Gregory Haskins <> | Subject | [PATCH v3] slow-work: add (module*)work->owner to fix races with module clients | Date | Wed, 24 Jun 2009 21:49:22 -0400 |
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(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 ]
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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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