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>

2 files changed, 3 insertions, 3 deletions

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 1ff76acc7e98..d1ed67145cf3 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -4150,7 +4150,7 @@ static void nfs4_lock_release(void *calldata)
 		task = nfs4_do_unlck(&data->fl, data->ctx, data->lsp,
 				data->arg.lock_seqid);
 		if (!IS_ERR(task))
-			rpc_put_task(task);
+			rpc_put_task_async(task);
 		dprintk("%s: cancelling lock!\n", __func__);
 	} else
 		nfs_free_seqid(data->arg.lock_seqid);
@@ -5227,7 +5227,7 @@ static int nfs41_proc_async_sequence(struct nfs_client *clp, struct rpc_cred *cr
 	if (IS_ERR(task))
 		ret = PTR_ERR(task);
 	else
-		rpc_put_task(task);
+		rpc_put_task_async(task);
 	dprintk("<-- %s status=%d\n", __func__, ret);
 	return ret;
 }
diff --git a/fs/nfs/unlink.c b/fs/nfs/unlink.c
index e313a51acdd1..6481d537d69d 100644
--- a/fs/nfs/unlink.c
+++ b/fs/nfs/unlink.c
@@ -180,7 +180,7 @@ static int nfs_do_call_unlink(struct dentry *parent, struct inode *dir, struct n
 	task_setup_data.rpc_client = NFS_CLIENT(dir);
 	task = rpc_run_task(&task_setup_data);
 	if (!IS_ERR(task))
-		rpc_put_task(task);
+		rpc_put_task_async(task);
 	return 1;
 }