SUNRPC: Close a race in __rpc_wait_for_completion_task()

Although they run as rpciod background tasks, under normal operation
(i.e. no SIGKILL), functions like nfs_sillyrename(), nfs4_proc_unlck()
and nfs4_do_close() want to be fully synchronous. This means that when we
exit, we want all references to the rpc_task to be gone, and we want
any dentry references etc. held by that task to be released.

For this reason these functions call __rpc_wait_for_completion_task(),
followed by rpc_put_task() in the expectation that the latter will be
releasing the last reference to the rpc_task, and thus ensuring that the
callback_ops->rpc_release() has been called synchronously.

This patch fixes a race which exists due to the fact that
rpciod calls rpc_complete_task() (in order to wake up the callers of
__rpc_wait_for_completion_task()) and then subsequently calls
rpc_put_task() without ensuring that these two steps are done atomically.

In order to avoid adding new spin locks, the patch uses the existing
waitqueue spin lock to order the rpc_task reference count releases between
the waiting process and rpciod.
The common case where nobody is waiting for completion is optimised for by
checking if the RPC_TASK_ASYNC flag is cleared and/or if the rpc_task
reference count is 1: in those cases we drop trying to grab the spin lock,
and immediately free up the rpc_task.

Those few processes that need to put the rpc_task from inside an
asynchronous context and that do not care about ordering are given a new
helper: rpc_put_task_async().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 61 insertions, 14 deletions

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 243fc09b164e..59e599498e37 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -252,23 +252,37 @@ static void rpc_set_active(struct rpc_task *task)
 
 /*
  * Mark an RPC call as having completed by clearing the 'active' bit
+ * and then waking up all tasks that were sleeping.
  */
-static void rpc_mark_complete_task(struct rpc_task *task)
+static int rpc_complete_task(struct rpc_task *task)
 {
-        smp_mb__before_clear_bit();
+        void *m = &task->tk_runstate;
+        wait_queue_head_t *wq = bit_waitqueue(m, RPC_TASK_ACTIVE);
+        struct wait_bit_key k = __WAIT_BIT_KEY_INITIALIZER(m, RPC_TASK_ACTIVE);
+        unsigned long flags;
+        int ret;
+
+        spin_lock_irqsave(&wq->lock, flags);
         clear_bit(RPC_TASK_ACTIVE, &task->tk_runstate);
-        smp_mb__after_clear_bit();
-        wake_up_bit(&task->tk_runstate, RPC_TASK_ACTIVE);
+        ret = atomic_dec_and_test(&task->tk_count);
+        if (waitqueue_active(wq))
+                __wake_up_locked_key(wq, TASK_NORMAL, &k);
+        spin_unlock_irqrestore(&wq->lock, flags);
+        return ret;
 }
 
 /*
  * Allow callers to wait for completion of an RPC call
+ *
+ * Note the use of out_of_line_wait_on_bit() rather than wait_on_bit()
+ * to enforce taking of the wq->lock and hence avoid races with
+ * rpc_complete_task().
  */
 int __rpc_wait_for_completion_task(struct rpc_task *task, int (*action)(void *))
 {
         if (action == NULL)
                 action = rpc_wait_bit_killable;
-        return wait_on_bit(&task->tk_runstate, RPC_TASK_ACTIVE,
+        return out_of_line_wait_on_bit(&task->tk_runstate, RPC_TASK_ACTIVE,
                         action, TASK_KILLABLE);
 }
 EXPORT_SYMBOL_GPL(__rpc_wait_for_completion_task);
@@ -857,34 +871,67 @@ static void rpc_async_release(struct work_struct *work)
         rpc_free_task(container_of(work, struct rpc_task, u.tk_work));
 }
 
-void rpc_put_task(struct rpc_task *task)
+static void rpc_release_resources_task(struct rpc_task *task)
 {
-        if (!atomic_dec_and_test(&task->tk_count))
-                return;
-        /* Release resources */
         if (task->tk_rqstp)
                 xprt_release(task);
         if (task->tk_msg.rpc_cred)
                 put_rpccred(task->tk_msg.rpc_cred);
         rpc_task_release_client(task);
-        if (task->tk_workqueue != NULL) {
+}
+
+static void rpc_final_put_task(struct rpc_task *task,
+                struct workqueue_struct *q)
+{
+        if (q != NULL) {
                 INIT_WORK(&task->u.tk_work, rpc_async_release);
-                queue_work(task->tk_workqueue, &task->u.tk_work);
+                queue_work(q, &task->u.tk_work);
         } else
                 rpc_free_task(task);
 }
+
+static void rpc_do_put_task(struct rpc_task *task, struct workqueue_struct *q)
+{
+        if (atomic_dec_and_test(&task->tk_count)) {
+                rpc_release_resources_task(task);
+                rpc_final_put_task(task, q);
+        }
+}
+
+void rpc_put_task(struct rpc_task *task)
+{
+        rpc_do_put_task(task, NULL);
+}
 EXPORT_SYMBOL_GPL(rpc_put_task);
 
+void rpc_put_task_async(struct rpc_task *task)
+{
+        rpc_do_put_task(task, task->tk_workqueue);
+}
+EXPORT_SYMBOL_GPL(rpc_put_task_async);
+
 static void rpc_release_task(struct rpc_task *task)
 {
         dprintk("RPC: %5u release task\n", task->tk_pid);
 
         BUG_ON (RPC_IS_QUEUED(task));
 
-        /* Wake up anyone who is waiting for task completion */
-        rpc_mark_complete_task(task);
+        rpc_release_resources_task(task);
 
-        rpc_put_task(task);
+        /*
+         * Note: at this point we have been removed from rpc_clnt->cl_tasks,
+         * so it should be safe to use task->tk_count as a test for whether
+         * or not any other processes still hold references to our rpc_task.
+         */
+        if (atomic_read(&task->tk_count) != 1 + !RPC_IS_ASYNC(task)) {
+                /* Wake up anyone who may be waiting for task completion */
+                if (!rpc_complete_task(task))
+                        return;
+        } else {
+                if (!atomic_dec_and_test(&task->tk_count))
+                        return;
+        }
+        rpc_final_put_task(task, task->tk_workqueue);
 }
 
 int rpciod_up(void)