[PATCH v4] slow-work: add (module*)work->ops->owner to fix races with module clients

Gregory Haskins
2009-06-29T19:24:18Z

(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.

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;
}