NFS: Add an iocounter wait function for async RPC tasks

By sleeping on a new NFS Unlock-On-Close waitqueue, rpc tasks may wait for
a lock context's iocounter to reach zero. The rpc waitqueue is only woken
when the open_context has the NFS_CONTEXT_UNLOCK flag set in order to
mitigate spurious wake-ups for any iocounter reaching zero.

Signed-off-by: Benjamin Coddington <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/client.c

···
 static void pnfs_init_server(struct nfs_server *server)
 {
 	rpc_init_wait_queue(&server->roc_rpcwaitq, "pNFS ROC");
+	rpc_init_wait_queue(&server->uoc_rpcwaitq, "NFS UOC");
 }
 
 #else

fs/nfs/pagelist.c

···
 			TASK_KILLABLE);
 }
 
+/**
+ * nfs_async_iocounter_wait - wait on a rpc_waitqueue for I/O
+ * to complete
+ * @task: the rpc_task that should wait
+ * @l_ctx: nfs_lock_context with io_counter to check
+ *
+ * Returns true if there is outstanding I/O to wait on and the
+ * task has been put to sleep.
+ */
+bool
+nfs_async_iocounter_wait(struct rpc_task *task, struct nfs_lock_context *l_ctx)
+{
+	struct inode *inode = d_inode(l_ctx->open_context->dentry);
+	bool ret = false;
+
+	if (atomic_read(&l_ctx->io_count) > 0) {
+		rpc_sleep_on(&NFS_SERVER(inode)->uoc_rpcwaitq, task, NULL);
+		ret = true;
+	}
+
+	if (atomic_read(&l_ctx->io_count) == 0) {
+		rpc_wake_up_queued_task(&NFS_SERVER(inode)->uoc_rpcwaitq, task);
+		ret = false;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(nfs_async_iocounter_wait);
+
 /*
  * nfs_page_group_lock - lock the head of the page group
  * @req - request in group that is to be locked
···
 		req->wb_page = NULL;
 	}
 	if (l_ctx != NULL) {
-		if (atomic_dec_and_test(&l_ctx->io_count))
+		if (atomic_dec_and_test(&l_ctx->io_count)) {
 			wake_up_atomic_t(&l_ctx->io_count);
+			if (test_bit(NFS_CONTEXT_UNLOCK, &ctx->flags))
+				rpc_wake_up(&NFS_SERVER(d_inode(ctx->dentry))->uoc_rpcwaitq);
+		}
 		nfs_put_lock_context(l_ctx);
 		req->wb_lock_context = NULL;
 	}

include/linux/nfs_fs.h

···
 #define NFS_CONTEXT_ERROR_WRITE		(0)
 #define NFS_CONTEXT_RESEND_WRITES	(1)
 #define NFS_CONTEXT_BAD			(2)
+#define NFS_CONTEXT_UNLOCK	(3)
 	int error;
 
 	struct list_head list;

include/linux/nfs_fs_sb.h

···
 	u32			mountd_version;
 	unsigned short		mountd_port;
 	unsigned short		mountd_protocol;
+	struct rpc_wait_queue	uoc_rpcwaitq;
 };
 
 /* Server capabilities */

include/linux/nfs_page.h

···
 extern void nfs_page_group_lock_wait(struct nfs_page *);
 extern void nfs_page_group_unlock(struct nfs_page *);
 extern bool nfs_page_group_sync_on_bit(struct nfs_page *, unsigned int);
+extern bool nfs_async_iocounter_wait(struct rpc_task *, struct nfs_lock_context *);
 
 /*
  * Lock the page of an asynchronous request