ceph: use empty snap context for uninline_data and get_pool_perm

Cached_context in ceph_snap_realm is directly accessed by
uninline_data() and get_pool_perm(). This is racy in theory.
both uninline_data() and get_pool_perm() do not modify existing
object, they only create new object. So we can pass the empty
snap context to them.  Unlike cached_context in ceph_snap_realm,
we do not need to protect the empty snap context.
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/addr.c

@@ -1510,8 +1510,7 @@ int ceph_uninline_data(struct file *filp, struct page *locked_page)
 				    ceph_vino(inode), 0, &len, 0, 1,
 				    CEPH_OSD_OP_CREATE,
 				    CEPH_OSD_FLAG_ONDISK | CEPH_OSD_FLAG_WRITE,
-				    ci->i_snap_realm->cached_context,
-				    0, 0, false);
+				    ceph_empty_snapc, 0, 0, false);
 	if (IS_ERR(req)) {
 		err = PTR_ERR(req);
 		goto out;
@@ -1529,7 +1528,7 @@ int ceph_uninline_data(struct file *filp, struct page *locked_page)
 				    ceph_vino(inode), 0, &len, 1, 3,
 				    CEPH_OSD_OP_WRITE,
 				    CEPH_OSD_FLAG_ONDISK | CEPH_OSD_FLAG_WRITE,
-				    ci->i_snap_realm->cached_context,
+				    ceph_empty_snapc,
 				    ci->i_truncate_seq, ci->i_truncate_size,
 				    false);
 	if (IS_ERR(req)) {
@@ -1653,7 +1652,7 @@ static int __ceph_pool_perm_get(struct ceph_inode_info *ci, u32 pool)
 	}
 
 	rd_req = ceph_osdc_alloc_request(&fsc->client->osdc,
-					 ci->i_snap_realm->cached_context,
+					 ceph_empty_snapc,
 					 1, false, GFP_NOFS);
 	if (!rd_req) {
 		err = -ENOMEM;
@@ -1668,7 +1667,7 @@ static int __ceph_pool_perm_get(struct ceph_inode_info *ci, u32 pool)
 	rd_req->r_base_oid.name_len = strlen(rd_req->r_base_oid.name);
 
 	wr_req = ceph_osdc_alloc_request(&fsc->client->osdc,
-					 ci->i_snap_realm->cached_context,
+					 ceph_empty_snapc,
 					 1, false, GFP_NOFS);
 	if (!wr_req) {
 		err = -ENOMEM;

fs/ceph/snap.c

@@ -296,7 +296,7 @@ static int cmpu64_rev(const void *a, const void *b)
 }
 
 
-static struct ceph_snap_context *empty_snapc;
+struct ceph_snap_context *ceph_empty_snapc;
 
 /*
  * build the snap context for a given realm.
@@ -338,9 +338,9 @@ static int build_snap_context(struct ceph_snap_realm *realm)
 		return 0;
 	}
 
-	if (num == 0 && realm->seq == empty_snapc->seq) {
-		ceph_get_snap_context(empty_snapc);
-		snapc = empty_snapc;
+	if (num == 0 && realm->seq == ceph_empty_snapc->seq) {
+		ceph_get_snap_context(ceph_empty_snapc);
+		snapc = ceph_empty_snapc;
 		goto done;
 	}
 
@@ -482,7 +482,7 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 		   cap_snap.  lucky us. */
 		dout("queue_cap_snap %p already pending\n", inode);
 		kfree(capsnap);
-	} else if (ci->i_snap_realm->cached_context == empty_snapc) {
+	} else if (ci->i_snap_realm->cached_context == ceph_empty_snapc) {
 		dout("queue_cap_snap %p empty snapc\n", inode);
 		kfree(capsnap);
 	} else if (dirty & (CEPH_CAP_AUTH_EXCL|CEPH_CAP_XATTR_EXCL|
@@ -964,14 +964,14 @@ void ceph_handle_snap(struct ceph_mds_client *mdsc,
 
 int __init ceph_snap_init(void)
 {
-	empty_snapc = ceph_create_snap_context(0, GFP_NOFS);
-	if (!empty_snapc)
+	ceph_empty_snapc = ceph_create_snap_context(0, GFP_NOFS);
+	if (!ceph_empty_snapc)
 		return -ENOMEM;
-	empty_snapc->seq = 1;
+	ceph_empty_snapc->seq = 1;
 	return 0;
 }
 
 void ceph_snap_exit(void)
 {
-	ceph_put_snap_context(empty_snapc);
+	ceph_put_snap_context(ceph_empty_snapc);
 }

fs/ceph/super.h

@@ -692,6 +692,7 @@ static inline int default_congestion_kb(void)
 
 
 /* snap.c */
+extern struct ceph_snap_context *ceph_empty_snapc;
 struct ceph_snap_realm *ceph_lookup_snap_realm(struct ceph_mds_client *mdsc,
 					       u64 ino);
 extern void ceph_get_snap_realm(struct ceph_mds_client *mdsc,