SUNRPC: Tighten up the task locking rules in __rpc_execute()

We should probably not be testing any flags after we've cleared the
RPC_TASK_RUNNING flag, since rpc_make_runnable() is then free to assign the
rpc_task to another workqueue, which may then destroy it.

We can fix any races with rpc_make_runnable() by ensuring that we only
clear the RPC_TASK_RUNNING flag while holding the rpc_wait_queue->lock that
the task is supposed to be sleeping on (and then checking whether or not
the task really is sleeping).

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

1 files changed, 20 insertions, 13 deletions

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 385f427beda..ff50a054686 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -293,11 +293,6 @@ static void rpc_make_runnable(struct rpc_task *task)
         rpc_clear_queued(task);
         if (rpc_test_and_set_running(task))
                 return;
-        /* We might have raced */
-        if (RPC_IS_QUEUED(task)) {
-                rpc_clear_running(task);
-                return;
-        }
         if (RPC_IS_ASYNC(task)) {
                 int status;
 
@@ -607,7 +602,9 @@ void rpc_release_calldata(const struct rpc_call_ops *ops, void *calldata)
  */
 static void __rpc_execute(struct rpc_task *task)
 {
-        int             status = 0;
+        struct rpc_wait_queue *queue;
+        int task_is_async = RPC_IS_ASYNC(task);
+        int status = 0;
 
         dprintk("RPC: %5u __rpc_execute flags=0x%x\n",
                         task->tk_pid, task->tk_flags);
@@ -647,15 +644,25 @@ static void __rpc_execute(struct rpc_task *task)
                  */
                 if (!RPC_IS_QUEUED(task))
                         continue;
-                rpc_clear_running(task);
-                if (RPC_IS_ASYNC(task)) {
-                        /* Careful! we may have raced... */
-                        if (RPC_IS_QUEUED(task))
-                                return;
-                        if (rpc_test_and_set_running(task))
-                                return;
+                /*
+                 * The queue->lock protects against races with
+                 * rpc_make_runnable().
+                 *
+                 * Note that once we clear RPC_TASK_RUNNING on an asynchronous
+                 * rpc_task, rpc_make_runnable() can assign it to a
+                 * different workqueue. We therefore cannot assume that the
+                 * rpc_task pointer may still be dereferenced.
+                 */
+                queue = task->tk_waitqueue;
+                spin_lock_bh(&queue->lock);
+                if (!RPC_IS_QUEUED(task)) {
+                        spin_unlock_bh(&queue->lock);
                         continue;
                 }
+                rpc_clear_running(task);
+                spin_unlock_bh(&queue->lock);
+                if (task_is_async)
+                        return;
 
                 /* sync task: sleep here */
                 dprintk("RPC: %5u sync task going to sleep\n", task->tk_pid);