[PATCH] Make sure sunrpc/cache doesn't confuse writers with readers.

When a sunrpc/cache channel is not open for reading, the
cache doesn't bother making and waiting for up-calls.

However it doesn't currently distingish between open-for-read/write
and open-for-write, so an op-for-write will look like a reader
and will cause inappropriate waiting.

This patch checks if a file is open-for-read and will only register
a file as a reader if it really is one.

net/sunrpc/cache.c

@@ -738,7 +738,9 @@ cache_ioctl(struct inode *ino, struct file *filp,
 static int
 cache_open(struct inode *inode, struct file *filp)
 {
-	struct cache_reader *rp;
+	struct cache_reader *rp = NULL;
+
+	if (filp->f_mode & FMODE_READ) {
 		struct cache_detail *cd = PDE(inode)->data;
 
 		rp = kmalloc(sizeof(*rp), GFP_KERNEL);
@@ -751,6 +753,7 @@ cache_open(struct inode *inode, struct file *filp)
 		spin_lock(&queue_lock);
 		list_add(&rp->q.list, &cd->queue);
 		spin_unlock(&queue_lock);
+	}
 	filp->private_data = rp;
 	return 0;
 }
@@ -761,6 +764,7 @@ cache_release(struct inode *inode, struct file *filp)
 	struct cache_reader *rp = filp->private_data;
 	struct cache_detail *cd = PDE(inode)->data;
 
+	if (rp) {
 		spin_lock(&queue_lock);
 		if (rp->offset) {
 			struct cache_queue *cq;
@@ -784,6 +788,7 @@ cache_release(struct inode *inode, struct file *filp)
 
 		cd->last_close = get_seconds();
 		atomic_dec(&cd->readers);
+	}
 	return 0;
 }