NFS: Fix nfs_page_group_destroy() and nfs_lock_and_join_requests() race cases

Since nfs_page_group_destroy() does not take any locks on the requests
to be freed, we need to ensure that we don't inadvertently free the
request in nfs_destroy_unlinked_subrequests() while the last reference
is being released elsewhere.

Do this by:

1) Taking a reference to the request unless it is already being freed
2) Checking (under the page group lock) if PG_TEARDOWN is already set before
   freeing an unreferenced request in nfs_destroy_unlinked_subrequests()

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/write.c

@@ -384,10 +384,11 @@ nfs_unroll_locks(struct inode *inode, struct nfs_page *head,
 	struct nfs_page *tmp;
 
 	/* relinquish all the locks successfully grabbed this run */
-	for (tmp = head->wb_this_page ; tmp != req; tmp = tmp->wb_this_page)
-		nfs_unlock_request(tmp);
-
-	WARN_ON_ONCE(test_bit(PG_TEARDOWN, &req->wb_flags));
+	for (tmp = head->wb_this_page ; tmp != req; tmp = tmp->wb_this_page) {
+		if (!kref_read(&tmp->wb_kref))
+			continue;
+		nfs_unlock_and_release_request(tmp);
+	}
 }
 
 /*
@@ -414,36 +415,32 @@ nfs_destroy_unlinked_subrequests(struct nfs_page *destroy_list,
 		WARN_ON_ONCE(old_head != subreq->wb_head);
 
 		/* make sure old group is not used */
-		subreq->wb_head = subreq;
 		subreq->wb_this_page = subreq;
 
-		/* subreq is now totally disconnected from page group or any
-		 * write / commit lists. last chance to wake any waiters */
-		nfs_unlock_request(subreq);
-
-		if (!test_bit(PG_TEARDOWN, &subreq->wb_flags)) {
-			/* release ref on old head request */
-			nfs_release_request(old_head);
+		/* Note: races with nfs_page_group_destroy() */
+		if (!kref_read(&subreq->wb_kref)) {
+			bool freeme = test_bit(PG_TEARDOWN, &subreq->wb_flags);
 
 			nfs_page_group_clear_bits(subreq);
+			/* Check if we raced with nfs_page_group_destroy() */
+			if (freeme)
+				nfs_free_request(subreq);
+			continue;
+		}
+
+		subreq->wb_head = subreq;
 
-			/* release the PG_INODE_REF reference */
 		if (test_and_clear_bit(PG_INODE_REF, &subreq->wb_flags)) {
 			nfs_release_request(subreq);
 			spin_lock(&inode->i_lock);
 			NFS_I(inode)->nrequests--;
 			spin_unlock(&inode->i_lock);
-			} else
-				WARN_ON_ONCE(1);
-		} else {
-			WARN_ON_ONCE(test_bit(PG_CLEAN, &subreq->wb_flags));
-			/* zombie requests have already released the last
-			 * reference and were waiting on the rest of the
-			 * group to complete. Since it's no longer part of a
-			 * group, simply free the request */
-			nfs_page_group_clear_bits(subreq);
-			nfs_free_request(subreq);
 		}
+
+		nfs_page_group_clear_bits(subreq);
+		/* subreq is now totally disconnected from page group or any
+		 * write / commit lists. last chance to wake any waiters */
+		nfs_unlock_and_release_request(subreq);
 	}
 }
 
@@ -512,6 +509,8 @@ try_again:
 	for (subreq = head->wb_this_page; subreq != head;
 			subreq = subreq->wb_this_page) {
 
+		if (!kref_get_unless_zero(&subreq->wb_kref))
+			continue;
 		while (!nfs_lock_request(subreq)) {
 			/*
 			 * Unlock page to allow nfs_page_group_sync_on_bit()
@@ -523,6 +522,7 @@ try_again:
 				ret = nfs_page_group_lock(head, false);
 			if (ret < 0) {
 				nfs_unroll_locks(inode, head, subreq);
+				nfs_release_request(subreq);
 				nfs_unlock_and_release_request(head);
 				return ERR_PTR(ret);
 			}
@@ -537,8 +537,8 @@ try_again:
 		} else if (WARN_ON_ONCE(subreq->wb_offset < head->wb_offset ||
 			    ((subreq->wb_offset + subreq->wb_bytes) >
 			     (head->wb_offset + total_bytes)))) {
-			nfs_unlock_request(subreq);
 			nfs_unroll_locks(inode, head, subreq);
+			nfs_unlock_and_release_request(subreq);
 			nfs_page_group_unlock(head);
 			nfs_unlock_and_release_request(head);
 			return ERR_PTR(-EIO);