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>

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 385f427..ff50a05 100644 (file)
--- 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);