[RFC PATCH v8 13/24] ceph: make ceph_msdc_build_path use ref-walk

[Jeff Layton <jlayton@kernel.org>]

Encryption potentially requires allocation, at which point we'll need to
be in a non-atomic context. Convert ceph_msdc_build_path to take dentry
spinlocks and references instead of using rcu_read_lock to walk the
path.

This is slightly less efficient, and we may want to eventually allow
using RCU when the leaf dentry isn't encrypted.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/ceph/mds_client.c | 35 +++++++++++++++++++----------------
 1 file changed, 19 insertions(+), 16 deletions(-)

diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 058480708897..518a1aee967c 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -2487,7 +2487,8 @@ static inline  u64 __get_oldest_tid(struct ceph_mds_client *mdsc)
 char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *pbase,
 			   int stop_on_nosnap)
 {
-	struct dentry *temp;
+	struct dentry *cur;
+	struct inode *inode;
 	char *path;
 	int pos;
 	unsigned seq;
@@ -2504,34 +2505,35 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *pbase,
 	path[pos] = '\0';
 
 	seq = read_seqbegin(&rename_lock);
-	rcu_read_lock();
-	temp = dentry;
+	cur = dget(dentry);
 	for (;;) {
-		struct inode *inode;
+		struct dentry *temp;
 
-		spin_lock(&temp->d_lock);
-		inode = d_inode(temp);
+		spin_lock(&cur->d_lock);
+		inode = d_inode(cur);
 		if (inode && ceph_snap(inode) == CEPH_SNAPDIR) {
 			dout("build_path path+%d: %p SNAPDIR\n",
-			     pos, temp);
-		} else if (stop_on_nosnap && inode && dentry != temp &&
+			     pos, cur);
+		} else if (stop_on_nosnap && inode && dentry != cur &&
 			   ceph_snap(inode) == CEPH_NOSNAP) {
-			spin_unlock(&temp->d_lock);
+			spin_unlock(&cur->d_lock);
 			pos++; /* get rid of any prepended '/' */
 			break;
 		} else {
-			pos -= temp->d_name.len;
+			pos -= cur->d_name.len;
 			if (pos < 0) {
-				spin_unlock(&temp->d_lock);
+				spin_unlock(&cur->d_lock);
 				break;
 			}
-			memcpy(path + pos, temp->d_name.name, temp->d_name.len);
+			memcpy(path + pos, cur->d_name.name, cur->d_name.len);
 		}
+		temp = cur;
 		spin_unlock(&temp->d_lock);
-		temp = READ_ONCE(temp->d_parent);
+		cur = dget_parent(temp);
+		dput(temp);
 
 		/* Are we at the root? */
-		if (IS_ROOT(temp))
+		if (IS_ROOT(cur))
 			break;
 
 		/* Are we out of buffer? */
@@ -2540,8 +2542,9 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *pbase,
 
 		path[pos] = '/';
 	}
-	base = ceph_ino(d_inode(temp));
-	rcu_read_unlock();
+	inode = d_inode(cur);
+	base = inode ? ceph_ino(inode) : 0;
+	dput(cur);
 
 	if (read_seqretry(&rename_lock, seq))
 		goto retry;
-- 
2.31.1