Messages in this thread Patch in this message | | | Date | Wed, 24 Jun 2009 15:33:23 -0400 | From | Gregory Haskins <> | Subject | [RFC PATCH] slow-work: add (module*)work->owner to fix races with module clients |
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(Applies to Linus' git master:626f380d)
Hi All, I found this while working on KVM. I actually posted this patch with a KVM series yesterday and standalone earlier today, but neither seems to have made it to the lists. I suspect there is an issue with git-mail/postfix on my system.
I digress. This is a repost with the patch by itself, and rebased to Linus' tree instead of kvm.git. Apologies if the system finally corrects itself and the others show up.
Thoughts?
Regards, -Greg
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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 transparently mapping "struct module* owner" to the slow_work 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> ---
include/linux/slow-work.h | 4 ++++ kernel/slow-work.c | 6 ++++++ 2 files changed, 10 insertions(+), 0 deletions(-)
diff --git a/include/linux/slow-work.h b/include/linux/slow-work.h index b65c888..9f48dab 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; @@ -42,6 +43,7 @@ struct slow_work_ops { * queued */ struct slow_work { + struct module *owner; unsigned long flags; #define SLOW_WORK_PENDING 0 /* item pending (further) execution */ #define SLOW_WORK_EXECUTING 1 /* item currently executing */ @@ -61,6 +63,7 @@ struct slow_work { static inline void slow_work_init(struct slow_work *work, const struct slow_work_ops *ops) { + work->owner = THIS_MODULE; work->flags = 0; work->ops = ops; INIT_LIST_HEAD(&work->link); @@ -78,6 +81,7 @@ static inline void slow_work_init(struct slow_work *work, static inline void vslow_work_init(struct slow_work *work, const struct slow_work_ops *ops) { + work->owner = THIS_MODULE; work->flags = 1 << SLOW_WORK_VERY_SLOW; work->ops = ops; INIT_LIST_HEAD(&work->link); diff --git a/kernel/slow-work.c b/kernel/slow-work.c index 09d7519..1dc3486 100644 --- a/kernel/slow-work.c +++ b/kernel/slow-work.c @@ -220,6 +220,8 @@ static bool slow_work_execute(void) } work->ops->put_ref(work); + barrier(); /* ensure that put_ref is not re-ordered with module_put */ + module_put(work->owner); return true; auto_requeue: @@ -299,6 +301,8 @@ 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 { + if (!try_module_get(work->owner)) + goto cant_get_mod; if (work->ops->get_ref(work) < 0) goto cant_get_ref; if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) @@ -313,6 +317,8 @@ int slow_work_enqueue(struct slow_work *work) return 0; cant_get_ref: + module_put(work->owner); +cant_get_mod: spin_unlock_irqrestore(&slow_work_queue_lock, flags); return -EAGAIN; }
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