NFSv4: Convert nfs_lock_and_join_requests() to use nfs_page_find_head_request()

Hide the locking from nfs_lock_and_join_requests() so that we can
separate out the requirements for swapcache pages.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/write.c

@@ -154,6 +154,14 @@ static void nfs_context_set_write_error(struct nfs_open_context *ctx, int error)
 	set_bit(NFS_CONTEXT_ERROR_WRITE, &ctx->flags);
 }
 
+static struct nfs_page *
+nfs_page_private_request(struct page *page)
+{
+	if (!PagePrivate(page))
+		return NULL;
+	return (struct nfs_page *)page_private(page);
+}
+
 /*
  * nfs_page_find_head_request_locked - find head request associated with @page
  *
@@ -164,11 +172,10 @@ static void nfs_context_set_write_error(struct nfs_open_context *ctx, int error)
 static struct nfs_page *
 nfs_page_find_head_request_locked(struct nfs_inode *nfsi, struct page *page)
 {
-	struct nfs_page *req = NULL;
+	struct nfs_page *req;
 
-	if (PagePrivate(page))
-		req = (struct nfs_page *)page_private(page);
-	else if (unlikely(PageSwapCache(page)))
+	req = nfs_page_private_request(page);
+	if (!req && unlikely(PageSwapCache(page)))
 		req = nfs_page_search_commits_for_head_request_locked(nfsi,
 			page);
 
@@ -448,31 +455,29 @@ nfs_lock_and_join_requests(struct page *page)
 	int ret;
 
 try_again:
-	if (!(PagePrivate(page) || PageSwapCache(page)))
-		return NULL;
-	spin_lock(&inode->i_lock);
 	/*
 	 * A reference is taken only on the head request which acts as a
 	 * reference to the whole page group - the group will not be destroyed
 	 * until the head reference is released.
 	 */
-	head = nfs_page_find_head_request_locked(NFS_I(inode), page);
-
-	if (!head) {
-		spin_unlock(&inode->i_lock);
+	head = nfs_page_find_head_request(page);
+	if (!head)
 		return NULL;
-	}
 
 	/* lock the page head first in order to avoid an ABBA inefficiency */
 	if (!nfs_lock_request(head)) {
-		spin_unlock(&inode->i_lock);
 		ret = nfs_wait_on_request(head);
 		nfs_release_request(head);
 		if (ret < 0)
 			return ERR_PTR(ret);
 		goto try_again;
 	}
-	spin_unlock(&inode->i_lock);
+
+	/* Ensure that nobody removed the request before we locked it */
+	if (head != nfs_page_private_request(page) && !PageSwapCache(page)) {
+		nfs_unlock_and_release_request(head);
+		goto try_again;
+	}
 
 	ret = nfs_page_group_lock(head);
 	if (ret < 0) {
@@ -559,7 +564,7 @@ nfs_lock_and_join_requests(struct page *page)
 		return NULL;
 	}
 
-	/* still holds ref on head from nfs_page_find_head_request_locked
+	/* still holds ref on head from nfs_page_find_head_request
 	 * and still has lock on head from lock loop */
 	return head;
 }