[PATCH] libceph: change how "safe" callback is used

[Alex Elder <elder@inktank.com>]

(This is an alternative to another patch that Zheng Yan
posted, "ceph: add osd request to inode unsafe list in
advance".  As noted, he's reviewed this one and I think
it can therefore take the place of his.)

An osd request currently has two callbacks.  They inform the
initiator of the request when we've received confirmation for the
target osd that a request was received, and when the osd indicates
all changes described by the request are durable.

The only time the second callback is used is in the ceph file system
for a synchronous write.  There's a race that makes some handling of
this case unsafe.  This patch addresses this problem.  The error
handling for this callback is also kind of gross, and this patch
changes that as well.


In ceph_sync_write(), if a safe callback is requested we want to add
the request on the ceph inode's unsafe items list.  Because items on
this list must have their tid set (by ceph_osd_start_request()), the
request added *after* the call to that function returns.  The
problem with this is that there's a race between starting the
request and adding it to the unsafe items list; the request may
already be complete before ceph_sync_write() even begins to put it
on the list.


To address this, we change the way the "safe" callback is used.
Rather than just calling it when the request is "safe", we use it to
notify the initiator the bounds (start and end) of the period during
which the request is *unsafe*.  So the initiator gets notified just
before the request gets sent to the osd (when it is "unsafe"), and
again when it's known the results are durable (it's no longer
unsafe).  The first call will get made just before the request
message gets sent to the messenger for the first time, *before* the
osd client's request mutex gets dropped.

We then have this callback function insert the request on the ceph
inode's unsafe list when we're told the request is unsafe.  This
will avoid the race because this call will be made under protection
of the osd client's request mutex.  It also nicely groups the setup
and cleanup of the state associated with managing unsafe requests.

The name of the "safe" callback field is changed to "unsafe" rather
to better reflect its new purpose.  It has a Boolean "unsafe"
parameter to indicate whether the request is becoming unsafe or is
now safe.  Because the "msg" parameter wasn't used, we drop that.

This resolves the original problem reportedin:
    http://tracker.ceph.com/issues/4706

Reported-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yan, Zheng <zheng.z.yan@intel.com>
---
 fs/ceph/file.c                  |   52
+++++++++++++++++++++------------------
 include/linux/ceph/osd_client.h |    4 ++-
 net/ceph/osd_client.c           |    6 +++--
 3 files changed, 35 insertions(+), 27 deletions(-)

diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 1d8d430..044f3bf 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -446,19 +446,35 @@ done:
 }

 /*
- * Write commit callback, called if we requested both an ACK and
- * ONDISK commit reply from the OSD.
+ * Write commit request unsafe callback, called to tell us when a
+ * request is unsafe (that is, in flight--has been handed to the
+ * messenger to send to its target osd).  It is called again when
+ * we've received a response message indicating the request is
+ * "safe" (its CEPH_OSD_FLAG_ONDISK flag is set), or when a request
+ * is completed early (and unsuccessfully) due to a timeout or
+ * interrupt.
+ *
+ * This is used if we requested both an ACK and ONDISK commit reply
+ * from the OSD.
  */
-static void sync_write_commit(struct ceph_osd_request *req,
-			      struct ceph_msg *msg)
+static void ceph_sync_write_unsafe(struct ceph_osd_request *req, bool
unsafe)
 {
 	struct ceph_inode_info *ci = ceph_inode(req->r_inode);

-	dout("sync_write_commit %p tid %llu\n", req, req->r_tid);
-	spin_lock(&ci->i_unsafe_lock);
-	list_del_init(&req->r_unsafe_item);
-	spin_unlock(&ci->i_unsafe_lock);
-	ceph_put_cap_refs(ci, CEPH_CAP_FILE_WR);
+	dout("%s %p tid %llu %ssafe\n", __func__, req, req->r_tid,
+		unsafe ? "un" : "");
+	if (unsafe) {
+		ceph_get_cap_refs(ci, CEPH_CAP_FILE_WR);
+		spin_lock(&ci->i_unsafe_lock);
+		list_add_tail(&req->r_unsafe_item,
+			      &ci->i_unsafe_writes);
+		spin_unlock(&ci->i_unsafe_lock);
+	} else {
+		spin_lock(&ci->i_unsafe_lock);
+		list_del_init(&req->r_unsafe_item);
+		spin_unlock(&ci->i_unsafe_lock);
+		ceph_put_cap_refs(ci, CEPH_CAP_FILE_WR);
+	}
 }

 /*
@@ -570,7 +586,8 @@ more:

 		if ((file->f_flags & O_SYNC) == 0) {
 			/* get a second commit callback */
-			req->r_safe_callback = sync_write_commit;
+			req->r_unsafe_callback = ceph_sync_write_unsafe;
+			req->r_inode = inode;
 			own_pages = true;
 		}
 	}
@@ -581,21 +598,8 @@ more:
 	ceph_osdc_build_request(req, pos, snapc, vino.snap, &mtime);

 	ret = ceph_osdc_start_request(&fsc->client->osdc, req, false);
-	if (!ret) {
-		if (req->r_safe_callback) {
-			/*
-			 * Add to inode unsafe list only after we
-			 * start_request so that a tid has been assigned.
-			 */
-			spin_lock(&ci->i_unsafe_lock);
-			list_add_tail(&req->r_unsafe_item,
-				      &ci->i_unsafe_writes);
-			spin_unlock(&ci->i_unsafe_lock);
-			ceph_get_cap_refs(ci, CEPH_CAP_FILE_WR);
-		}
-		
+	if (!ret)
 		ret = ceph_osdc_wait_request(&fsc->client->osdc, req);
-	}

 	if (file->f_flags & O_DIRECT)
 		ceph_put_page_vector(pages, num_pages, false);
diff --git a/include/linux/ceph/osd_client.h
b/include/linux/ceph/osd_client.h
index 2a68a74..0d3358e 100644
--- a/include/linux/ceph/osd_client.h
+++ b/include/linux/ceph/osd_client.h
@@ -29,6 +29,7 @@ struct ceph_authorizer;
  */
 typedef void (*ceph_osdc_callback_t)(struct ceph_osd_request *,
 				     struct ceph_msg *);
+typedef void (*ceph_osdc_unsafe_callback_t)(struct ceph_osd_request *,
bool);

 /* a given osd we're communicating with */
 struct ceph_osd {
@@ -149,7 +150,8 @@ struct ceph_osd_request {
 	struct kref       r_kref;
 	bool              r_mempool;
 	struct completion r_completion, r_safe_completion;
-	ceph_osdc_callback_t r_callback, r_safe_callback;
+	ceph_osdc_callback_t r_callback;
+	ceph_osdc_unsafe_callback_t r_unsafe_callback;
 	struct ceph_eversion r_reassert_version;
 	struct list_head  r_unsafe_item;

diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
index 939be67..5b7ce57 100644
--- a/net/ceph/osd_client.c
+++ b/net/ceph/osd_client.c
@@ -1403,8 +1403,8 @@ static void handle_osds_timeout(struct work_struct
*work)

 static void complete_request(struct ceph_osd_request *req)
 {
-	if (req->r_safe_callback)
-		req->r_safe_callback(req, NULL);
+	if (req->r_unsafe_callback)
+		req->r_unsafe_callback(req, false);
 	complete_all(&req->r_safe_completion);  /* fsync waiter */
 }

@@ -2105,6 +2105,8 @@ int ceph_osdc_start_request(struct ceph_osd_client
*osdc,
 		dout("send_request %p no up osds in pg\n", req);
 		ceph_monc_request_next_osdmap(&osdc->client->monc);
 	} else {
+		if (req->r_unsafe_callback)
+			req->r_unsafe_callback(req, true);
 		__send_queued(osdc);
 	}
 	rc = 0;
-- 
1.7.9.5