ceph: set i_head_snapc when getting CEPH_CAP_FILE_WR reference

In most cases that snap context is needed, we are holding
reference of CEPH_CAP_FILE_WR. So we can set ceph inode's
i_head_snapc when getting the CEPH_CAP_FILE_WR reference,
and make codes get snap context from i_head_snapc. This makes
the code simpler.

Another benefit of this change is that we can handle snap
notification more elegantly. Especially when snap context
is updated while someone else is doing write. The old queue
cap_snap code may set cap_snap's context to ether the old
context or the new snap context, depending on if i_head_snapc
is set. The new queue capp_snap code always set cap_snap's
context to the old snap context.

Signed-off-by: Yan, Zheng <zyan@redhat.com>

1 files changed, 66 insertions, 60 deletions

diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index b2a9453..5bfdab9a 100644
--- a/fs/ceph/snap.c
+++ b/fs/ceph/snap.c
@@ -455,6 +455,7 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 {
 	struct inode *inode = &ci->vfs_inode;
 	struct ceph_cap_snap *capsnap;
+	struct ceph_snap_context *old_snapc;
 	int used, dirty;
 
 	capsnap = kzalloc(sizeof(*capsnap), GFP_NOFS);
@@ -467,6 +468,8 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 	used = __ceph_caps_used(ci);
 	dirty = __ceph_caps_dirty(ci);
 
+	old_snapc = ci->i_head_snapc;
+
 	/*
 	 * If there is a write in progress, treat that as a dirty Fw,
 	 * even though it hasn't completed yet; by the time we finish
@@ -481,76 +484,79 @@ void ceph_queue_cap_snap(struct ceph_inode_info *ci)
 		   writes in progress now were started before the previous
 		   cap_snap.  lucky us. */
 		dout("queue_cap_snap %p already pending\n", inode);
-		kfree(capsnap);
-	} else if (ci->i_snap_realm->cached_context == ceph_empty_snapc) {
+		goto update_snapc;
+	}
+	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|
-			    CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_WR)) {
-		struct ceph_snap_context *snapc = ci->i_head_snapc;
+		goto update_snapc;
+	}
+	if (!(dirty & (CEPH_CAP_AUTH_EXCL|CEPH_CAP_XATTR_EXCL|
+		       CEPH_CAP_FILE_EXCL|CEPH_CAP_FILE_WR))) {
+		dout("queue_cap_snap %p nothing dirty|writing\n", inode);
+		goto update_snapc;
+	}
 
-		/*
-		 * if we are a sync write, we may need to go to the snaprealm
-		 * to get the current snapc.
-		 */
-		if (!snapc)
-			snapc = ci->i_snap_realm->cached_context;
+	BUG_ON(!old_snapc);
 
-		dout("queue_cap_snap %p cap_snap %p queuing under %p %s\n",
-		     inode, capsnap, snapc, ceph_cap_string(dirty));
-		ihold(inode);
+	dout("queue_cap_snap %p cap_snap %p queuing under %p %s\n",
+	     inode, capsnap, old_snapc, ceph_cap_string(dirty));
+	ihold(inode);
 
-		atomic_set(&capsnap->nref, 1);
-		capsnap->ci = ci;
-		INIT_LIST_HEAD(&capsnap->ci_item);
-		INIT_LIST_HEAD(&capsnap->flushing_item);
-
-		capsnap->follows = snapc->seq;
-		capsnap->issued = __ceph_caps_issued(ci, NULL);
-		capsnap->dirty = dirty;
-
-		capsnap->mode = inode->i_mode;
-		capsnap->uid = inode->i_uid;
-		capsnap->gid = inode->i_gid;
-
-		if (dirty & CEPH_CAP_XATTR_EXCL) {
-			__ceph_build_xattrs_blob(ci);
-			capsnap->xattr_blob =
-				ceph_buffer_get(ci->i_xattrs.blob);
-			capsnap->xattr_version = ci->i_xattrs.version;
-		} else {
-			capsnap->xattr_blob = NULL;
-			capsnap->xattr_version = 0;
-		}
+	atomic_set(&capsnap->nref, 1);
+	capsnap->ci = ci;
+	INIT_LIST_HEAD(&capsnap->ci_item);
+	INIT_LIST_HEAD(&capsnap->flushing_item);
 
-		capsnap->inline_data = ci->i_inline_version != CEPH_INLINE_NONE;
+	capsnap->follows = old_snapc->seq;
+	capsnap->issued = __ceph_caps_issued(ci, NULL);
+	capsnap->dirty = dirty;
 
-		/* dirty page count moved from _head to this cap_snap;
-		   all subsequent writes page dirties occur _after_ this
-		   snapshot. */
-		capsnap->dirty_pages = ci->i_wrbuffer_ref_head;
-		ci->i_wrbuffer_ref_head = 0;
-		capsnap->context = snapc;
-		ci->i_head_snapc =
-			ceph_get_snap_context(ci->i_snap_realm->cached_context);
-		dout(" new snapc is %p\n", ci->i_head_snapc);
-		list_add_tail(&capsnap->ci_item, &ci->i_cap_snaps);
-
-		if (used & CEPH_CAP_FILE_WR) {
-			dout("queue_cap_snap %p cap_snap %p snapc %p"
-			     " seq %llu used WR, now pending\n", inode,
-			     capsnap, snapc, snapc->seq);
-			capsnap->writing = 1;
-		} else {
-			/* note mtime, size NOW. */
-			__ceph_finish_cap_snap(ci, capsnap);
-		}
+	capsnap->mode = inode->i_mode;
+	capsnap->uid = inode->i_uid;
+	capsnap->gid = inode->i_gid;
+
+	if (dirty & CEPH_CAP_XATTR_EXCL) {
+		__ceph_build_xattrs_blob(ci);
+		capsnap->xattr_blob =
+			ceph_buffer_get(ci->i_xattrs.blob);
+		capsnap->xattr_version = ci->i_xattrs.version;
 	} else {
-		dout("queue_cap_snap %p nothing dirty|writing\n", inode);
-		kfree(capsnap);
+		capsnap->xattr_blob = NULL;
+		capsnap->xattr_version = 0;
 	}
 
+	capsnap->inline_data = ci->i_inline_version != CEPH_INLINE_NONE;
+
+	/* dirty page count moved from _head to this cap_snap;
+	   all subsequent writes page dirties occur _after_ this
+	   snapshot. */
+	capsnap->dirty_pages = ci->i_wrbuffer_ref_head;
+	ci->i_wrbuffer_ref_head = 0;
+	capsnap->context = old_snapc;
+	list_add_tail(&capsnap->ci_item, &ci->i_cap_snaps);
+	old_snapc = NULL;
+
+	if (used & CEPH_CAP_FILE_WR) {
+		dout("queue_cap_snap %p cap_snap %p snapc %p"
+		     " seq %llu used WR, now pending\n", inode,
+		     capsnap, old_snapc, old_snapc->seq);
+		capsnap->writing = 1;
+	} else {
+		/* note mtime, size NOW. */
+		__ceph_finish_cap_snap(ci, capsnap);
+	}
+	capsnap = NULL;
+
+update_snapc:
+	if (ci->i_head_snapc) {
+		ci->i_head_snapc = ceph_get_snap_context(
+				ci->i_snap_realm->cached_context);
+		dout(" new snapc is %p\n", ci->i_head_snapc);
+	}
 	spin_unlock(&ci->i_ceph_lock);
+
+	kfree(capsnap);
+	ceph_put_snap_context(old_snapc);
 }
 
 /*