From: Christophe Saout Date: Sun, 05 Nov 2006 18:42:48 +0100 Subject: Re: [PATCH] Fix SUNRPC wakeup/execute race condition The sunrpc scheduler contains a race condition that can let an RPC task end up being neither running nor on any wait queue. The race takes place between rpc_make_runnable (called from rpc_wake_up_task) and __rpc_execute under the following condition: First __rpc_execute calls tk_action which puts the task on some wait queue. The task is dequeued by another process before __rpc_execute continues its execution. While executing rpc_make_runnable exactly after setting the task `running' bit and before clearing the `queued' bit __rpc_execute picks up execution, clears `running' and subsequently both functions fall through, both under the false assumption somebody else took the job. Swapping rpc_test_and_set_running with rpc_clear_queued in rpc_make_runnable fixes that hole. This introduces another possible race condition that can be handled by checking for `queued' after setting the `running' bit. Bug noticed on a 4-way x86_64 system under XEN with an NFSv4 server on the same physical machine, apparently one of the few ways to hit this race condition at all. Cc: Trond Myklebust Cc: J. Bruce Fields Signed-off-by: Christophe Saout Signed-off-by: Trond Myklebust --- net/sunrpc/sched.c | 10 ++++++---- 1 files changed, 6 insertions(+), 4 deletions(-) diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c index a1ab4ee..b57d406 100644 --- a/net/sunrpc/sched.c +++ b/net/sunrpc/sched.c @@ -295,13 +295,15 @@ EXPORT_SYMBOL(__rpc_wait_for_completion_ */ static void rpc_make_runnable(struct rpc_task *task) { - int do_ret; - BUG_ON(task->tk_timeout_fn); - do_ret = rpc_test_and_set_running(task); rpc_clear_queued(task); - if (do_ret) + 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;