From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758190AbZGGQFd (ORCPT ); Tue, 7 Jul 2009 12:05:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753502AbZGGQF0 (ORCPT ); Tue, 7 Jul 2009 12:05:26 -0400 Received: from mx2.redhat.com ([66.187.237.31]:48121 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754657AbZGGQFZ (ORCPT ); Tue, 7 Jul 2009 12:05:25 -0400 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells Subject: [PATCH] slow-work: Add (module*)work->ops->owner to fix races with module clients To: torvalds@osdl.org, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, Gregory Haskins , David Howells , Steven Whitehouse Date: Tue, 07 Jul 2009 17:04:19 +0100 Message-ID: <20090707160419.16976.34184.stgit@warthog.procyon.org.uk> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Gregory Haskins 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 Reviewed-by: Michael S. Tsirkin Signed-off-by: David Howells CC: Steven Whitehouse --- Documentation/slow-work.txt | 11 ++++++++++- 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, 35 insertions(+), 2 deletions(-) diff --git a/Documentation/slow-work.txt b/Documentation/slow-work.txt index ebc50f8..0a3faf6 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 @@ -141,6 +145,11 @@ All members are required: it. The thread pool will not touch the item again once this has been called. + This function must interpolate a general SMP memory barrier before freeing + or re-using the work struct as the caller may have read the module + pointer. Implying a barrier with something like atomic_dec_and_test() is + sufficient. + (*) Execute an item: void (*execute)(struct slow_work *work); 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 +#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; }