On Thu, 2015-07-30 at 06:07 -0700, Paul E. McKenney wrote:
> On Thu, Jul 30, 2015 at 06:15:23AM +0000, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
> >
> > This patch addresses a v4.2-rc1 regression where backend driver
> > struct module unload immediately after ->free_device() has done
> > an internal call_rcu(), results in IRQ rcu_process_callbacks()
> > use-after-free paging OOPsen.
> >
> > It adds a explicit synchronize_rcu() in target_backend_unregister()
> > to wait a full RCU grace period before releasing target_backend_ops
> > memory, and allowing TBO->module exit to proceed.
>
> Good catch, but...
>
> You need rcu_barrier() rather than synchronize_rcu() in this case.
> All that synchronize_rcu() does is wait for pre-existing RCU readers,
> when what is needed is to wait for all pre-existing RCU callbacks
> to be invoked.
>
Ah, was getting confused by rcu_barrier_tasks() being specific to
CONFIG_TASKS_RCU in update.c code, and missing rcu_barrier() in
tree_plugin.h.
Should have taken a look at Documentation/RCU/rcubarrier.txt..
> So please replace the two synchronize_rcu() calls with rcu_barrier().
>
<nod>, below is the updated version.
Thanks for the review!
--nab
>From 9721910116f9883c7afded613ec88dc2de12339a Mon Sep 17 00:00:00 2001
From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
Date: Wed, 29 Jul 2015 22:27:13 -0700
Subject: [PATCH] target: Perform RCU callback barrier before backend/fabric
unload
This patch addresses a v4.2-rc1 regression where backend driver
module unload happening immediately after TBO->free_device()
does internal call_rcu(), will currently result in IRQ context
rcu_process_callbacks() use-after-free paging OOPsen.
It adds the missing rcu_barrier() in target_backend_unregister()
to perform an explicit RCU barrier waiting for all RCU callbacks
to complete before releasing target_backend_ops memory, and
allowing TBO->module exit to proceed.
Also, do the same for fabric drivers in target_unregister_template()
to ensure se_deve_entry->rcu_head -> kfree_rcu() callbacks have
completed, before allowing target_core_fabric_ops->owner module
exit to proceed.
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Cc: Sagi Grimberg <sagig@xxxxxxxxxxxx>
Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
---
drivers/target/target_core_configfs.c | 9 ++++++++-
drivers/target/target_core_hba.c | 10 +++++++++-
2 files changed, 17 insertions(+), 2 deletions(-)
diff --git a/drivers/target/target_core_configfs.c b/drivers/target/target_core_configfs.c
index c2e9fea..860e840 100644
--- a/drivers/target/target_core_configfs.c
+++ b/drivers/target/target_core_configfs.c
@@ -457,8 +457,15 @@ void target_unregister_template(const struct target_core_fabric_ops *fo)
if (!strcmp(t->tf_ops->name, fo->name)) {
BUG_ON(atomic_read(&t->tf_access_cnt));
list_del(&t->tf_list);
+ mutex_unlock(&g_tf_lock);
+ /*
+ * Wait for any outstanding fabric se_deve_entry->rcu_head
+ * callbacks to complete post kfree_rcu(), before allowing
+ * fabric driver unload of TFO->module to proceed.
+ */
+ rcu_barrier();
kfree(t);
- break;
+ return;
}
}
mutex_unlock(&g_tf_lock);
diff --git a/drivers/target/target_core_hba.c b/drivers/target/target_core_hba.c
index 62ea4e8..be9cefc 100644
--- a/drivers/target/target_core_hba.c
+++ b/drivers/target/target_core_hba.c
@@ -84,8 +84,16 @@ void target_backend_unregister(const struct target_backend_ops *ops)
list_for_each_entry(tb, &backend_list, list) {
if (tb->ops == ops) {
list_del(&tb->list);
+ mutex_unlock(&backend_mutex);
+ /*
+ * Wait for any outstanding backend driver ->rcu_head
+ * callbacks to complete post TBO->free_device() ->
+ * call_rcu(), before allowing backend driver module
+ * unload of target_backend_ops->owner to proceed.
+ */
+ rcu_barrier();
kfree(tb);
- break;
+ return;
}
}
mutex_unlock(&backend_mutex);
--
1.9.1
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