ceph: take snap_rwsem when accessing snap realm's cached_context

When ceph inode's i_head_snapc is NULL, __ceph_mark_dirty_caps()
accesses snap realm's cached_context. So we need take read lock
of snap_rwsem.
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/caps.c

@@ -1413,9 +1413,11 @@ int __ceph_mark_dirty_caps(struct ceph_inode_info *ci, int mask)
 	     ceph_cap_string(was | mask));
 	ci->i_dirty_caps |= mask;
 	if (was == 0) {
-		if (!ci->i_head_snapc)
+		if (!ci->i_head_snapc) {
+			WARN_ON_ONCE(!rwsem_is_locked(&mdsc->snap_rwsem));
 			ci->i_head_snapc = ceph_get_snap_context(
 				ci->i_snap_realm->cached_context);
+		}
 		dout(" inode %p now dirty snapc %p auth cap %p\n",
 		     &ci->vfs_inode, ci->i_head_snapc, ci->i_auth_cap);
 		BUG_ON(!list_empty(&ci->i_dirty_item));

fs/ceph/inode.c

@@ -1727,6 +1727,7 @@ int ceph_setattr(struct dentry *dentry, struct iattr *attr)
 	int mask = 0;
 	int err = 0;
 	int inode_dirty_flags = 0;
+	bool lock_snap_rwsem = false;
 
 	if (ceph_snap(inode) != CEPH_NOSNAP)
 		return -EROFS;
@@ -1742,6 +1743,18 @@ int ceph_setattr(struct dentry *dentry, struct iattr *attr)
 
 	spin_lock(&ci->i_ceph_lock);
 	issued = __ceph_caps_issued(ci, NULL);
+
+	if (!ci->i_head_snapc &&
+	    (issued & (CEPH_CAP_ANY_EXCL | CEPH_CAP_FILE_WR))) {
+		lock_snap_rwsem = true;
+		if (!down_read_trylock(&mdsc->snap_rwsem)) {
+			spin_unlock(&ci->i_ceph_lock);
+			down_read(&mdsc->snap_rwsem);
+			spin_lock(&ci->i_ceph_lock);
+			issued = __ceph_caps_issued(ci, NULL);
+		}
+	}
+
 	dout("setattr %p issued %s\n", inode, ceph_cap_string(issued));
 
 	if (ia_valid & ATTR_UID) {
@@ -1890,6 +1903,8 @@ int ceph_setattr(struct dentry *dentry, struct iattr *attr)
 
 	release &= issued;
 	spin_unlock(&ci->i_ceph_lock);
+	if (lock_snap_rwsem)
+		up_read(&mdsc->snap_rwsem);
 
 	if (inode_dirty_flags)
 		__mark_inode_dirty(inode, inode_dirty_flags);

fs/ceph/xattr.c

@@ -911,6 +911,7 @@ int __ceph_setxattr(struct dentry *dentry, const char *name,
 	struct inode *inode = d_inode(dentry);
 	struct ceph_vxattr *vxattr;
 	struct ceph_inode_info *ci = ceph_inode(inode);
+	struct ceph_mds_client *mdsc = ceph_sb_to_client(dentry->d_sb)->mdsc;
 	int issued;
 	int err;
 	int dirty = 0;
@@ -920,6 +921,7 @@ int __ceph_setxattr(struct dentry *dentry, const char *name,
 	char *newval = NULL;
 	struct ceph_inode_xattr *xattr = NULL;
 	int required_blob_size;
+	bool lock_snap_rwsem = false;
 
 	if (!ceph_is_valid_xattr(name))
 		return -EOPNOTSUPP;
@@ -951,9 +953,20 @@ int __ceph_setxattr(struct dentry *dentry, const char *name,
 	spin_lock(&ci->i_ceph_lock);
 retry:
 	issued = __ceph_caps_issued(ci, NULL);
-	dout("setxattr %p issued %s\n", inode, ceph_cap_string(issued));
 	if (ci->i_xattrs.version == 0 || !(issued & CEPH_CAP_XATTR_EXCL))
 		goto do_sync;
+
+	if (!lock_snap_rwsem && !ci->i_head_snapc) {
+		lock_snap_rwsem = true;
+		if (!down_read_trylock(&mdsc->snap_rwsem)) {
+			spin_unlock(&ci->i_ceph_lock);
+			down_read(&mdsc->snap_rwsem);
+			spin_lock(&ci->i_ceph_lock);
+			goto retry;
+		}
+	}
+
+	dout("setxattr %p issued %s\n", inode, ceph_cap_string(issued));
 	__build_xattrs(inode);
 
 	required_blob_size = __get_required_blob_size(ci, name_len, val_len);
@@ -966,7 +979,7 @@ int __ceph_setxattr(struct dentry *dentry, const char *name,
 		dout(" preaallocating new blob size=%d\n", required_blob_size);
 		blob = ceph_buffer_new(required_blob_size, GFP_NOFS);
 		if (!blob)
-			goto out;
+			goto do_sync_unlocked;
 		spin_lock(&ci->i_ceph_lock);
 		if (ci->i_xattrs.prealloc_blob)
 			ceph_buffer_put(ci->i_xattrs.prealloc_blob);
@@ -984,6 +997,8 @@ int __ceph_setxattr(struct dentry *dentry, const char *name,
 	}
 
 	spin_unlock(&ci->i_ceph_lock);
+	if (lock_snap_rwsem)
+		up_read(&mdsc->snap_rwsem);
 	if (dirty)
 		__mark_inode_dirty(inode, dirty);
 	return err;
@@ -991,6 +1006,8 @@ int __ceph_setxattr(struct dentry *dentry, const char *name,
 do_sync:
 	spin_unlock(&ci->i_ceph_lock);
 do_sync_unlocked:
+	if (lock_snap_rwsem)
+		up_read(&mdsc->snap_rwsem);
 	err = ceph_sync_setxattr(dentry, name, value, size, flags);
 out:
 	kfree(newname);
@@ -1044,10 +1061,12 @@ int __ceph_removexattr(struct dentry *dentry, const char *name)
 	struct inode *inode = d_inode(dentry);
 	struct ceph_vxattr *vxattr;
 	struct ceph_inode_info *ci = ceph_inode(inode);
+	struct ceph_mds_client *mdsc = ceph_sb_to_client(dentry->d_sb)->mdsc;
 	int issued;
 	int err;
 	int required_blob_size;
 	int dirty;
+	bool lock_snap_rwsem = false;
 
 	if (!ceph_is_valid_xattr(name))
 		return -EOPNOTSUPP;
@@ -1064,10 +1083,21 @@ int __ceph_removexattr(struct dentry *dentry, const char *name)
 	spin_lock(&ci->i_ceph_lock);
 retry:
 	issued = __ceph_caps_issued(ci, NULL);
-	dout("removexattr %p issued %s\n", inode, ceph_cap_string(issued));
-
 	if (ci->i_xattrs.version == 0 || !(issued & CEPH_CAP_XATTR_EXCL))
 		goto do_sync;
+
+	if (!lock_snap_rwsem && !ci->i_head_snapc) {
+		lock_snap_rwsem = true;
+		if (!down_read_trylock(&mdsc->snap_rwsem)) {
+			spin_unlock(&ci->i_ceph_lock);
+			down_read(&mdsc->snap_rwsem);
+			spin_lock(&ci->i_ceph_lock);
+			goto retry;
+		}
+	}
+
+	dout("removexattr %p issued %s\n", inode, ceph_cap_string(issued));
+
 	__build_xattrs(inode);
 
 	required_blob_size = __get_required_blob_size(ci, 0, 0);
@@ -1080,7 +1110,7 @@ int __ceph_removexattr(struct dentry *dentry, const char *name)
 		dout(" preaallocating new blob size=%d\n", required_blob_size);
 		blob = ceph_buffer_new(required_blob_size, GFP_NOFS);
 		if (!blob)
-			goto out;
+			goto do_sync_unlocked;
 		spin_lock(&ci->i_ceph_lock);
 		if (ci->i_xattrs.prealloc_blob)
 			ceph_buffer_put(ci->i_xattrs.prealloc_blob);
@@ -1094,14 +1124,17 @@ int __ceph_removexattr(struct dentry *dentry, const char *name)
 	ci->i_xattrs.dirty = true;
 	inode->i_ctime = CURRENT_TIME;
 	spin_unlock(&ci->i_ceph_lock);
+	if (lock_snap_rwsem)
+		up_read(&mdsc->snap_rwsem);
 	if (dirty)
 		__mark_inode_dirty(inode, dirty);
 	return err;
 do_sync:
 	spin_unlock(&ci->i_ceph_lock);
 do_sync_unlocked:
+	if (lock_snap_rwsem)
+		up_read(&mdsc->snap_rwsem);
 	err = ceph_send_removexattr(dentry, name);
-out:
 	return err;
 }