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, 1 insertions, 0 deletions

diff --git a/include/linux/sunrpc/sched.h b/include/linux/sunrpc/sched.h
index 88513fd8e208..d81db8012c63 100644
--- a/include/linux/sunrpc/sched.h
+++ b/include/linux/sunrpc/sched.h
@@ -212,6 +212,7 @@ struct rpc_task *rpc_run_task(const struct rpc_task_setup *);
 struct rpc_task *rpc_run_bc_task(struct rpc_rqst *req,
                                 const struct rpc_call_ops *ops);
 void            rpc_put_task(struct rpc_task *);
+void            rpc_put_task_async(struct rpc_task *);
 void            rpc_exit_task(struct rpc_task *);
 void            rpc_exit(struct rpc_task *, int);
 void            rpc_release_calldata(const struct rpc_call_ops *, void *);