[xen master] tools/xenstored: Delay new transaction while Live-Update is pending

[patchbot@xxxxxxx]

commit c8be420025ada0f90e98ae0408226e29017757df
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Thu Jun 24 12:14:56 2021 +0100
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Thu Jun 24 12:14:56 2021 +0100

    tools/xenstored: Delay new transaction while Live-Update is pending
    
    At the moment, Live-Update will, by default, not proceed if there are
    in-flight transactions. It is possible force it by passing -F but this
    will break any connection with in-flight transactions.
    
    There are PV drivers out that may never terminate some transaction. On
    host running such guest, we would need to use -F. Unfortunately, this
    also risks to break well-behaving guests (and even dom0) because
    Live-Update will happen as soon as the timeout is hit.
    
    Ideally, we would want to preserve transactions but this requires
    some work and a lot of testing to be able to use it in production.
    
    As a stop gap, we want to limit the damage of -F. This patch will delay
    any transactions that are started after Live-Update has been requested.
    
    If the request cannot be delayed, the connection will be stalled to
    avoid loosing requests.
    
    If the connection has already a pending transaction before Live-Update,
    then new transaction will not be delayed. This is to avoid the connection
    to stall.
    
    With this stop gap in place, domains with long running transactions will
    still break when using -F, but other domains which starts a transaction
    in the middle of Live-Update will continue to work.
    
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
---
 tools/xenstore/xenstored_control.c | 10 ++++++++
 tools/xenstore/xenstored_control.h |  2 ++
 tools/xenstore/xenstored_core.c    | 49 +++++++++++++++++++++++++++++++++++++-
 tools/xenstore/xenstored_core.h    |  3 +++
 4 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/tools/xenstore/xenstored_control.c 
b/tools/xenstore/xenstored_control.c
index 1c24d4869e..a045f102a4 100644
--- a/tools/xenstore/xenstored_control.c
+++ b/tools/xenstore/xenstored_control.c
@@ -131,6 +131,11 @@ unsigned int lu_write_response(FILE *fp)
        return sizeof(msg) + msg.len;
 }
 
+bool lu_is_pending(void)
+{
+       return lu_status != NULL;
+}
+
 #else
 struct connection *lu_get_connection(void)
 {
@@ -142,6 +147,11 @@ unsigned int lu_write_response(FILE *fp)
        /* Unsupported */
        return 0;
 }
+
+bool lu_is_pending(void)
+{
+       return false;
+}
 #endif
 
 struct cmd_s {
diff --git a/tools/xenstore/xenstored_control.h 
b/tools/xenstore/xenstored_control.h
index 27d7f19e4b..98b6fbcea2 100644
--- a/tools/xenstore/xenstored_control.h
+++ b/tools/xenstore/xenstored_control.h
@@ -23,3 +23,5 @@ struct connection *lu_get_connection(void);
 
 /* Write the "OK" response for the live-update command */
 unsigned int lu_write_response(FILE *fp);
+
+bool lu_is_pending(void);
diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 0cb46d8c64..9ffd2ac66d 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -338,7 +338,20 @@ static int destroy_conn(void *_conn)
 
 static bool conn_can_read(struct connection *conn)
 {
-       return conn->funcs->can_read(conn) && !conn->is_ignored;
+       if (!conn->funcs->can_read(conn))
+               return false;
+
+       if (conn->is_ignored)
+               return false;
+
+       /*
+        * For stalled connection, we want to process the pending
+        * command as soon as live-update has aborted.
+        */
+       if (conn->is_stalled)
+               return !lu_is_pending();
+
+       return true;
 }
 
 static bool conn_can_write(struct connection *conn)
@@ -417,6 +430,12 @@ static void initialize_fds(int *p_sock_pollfd_idx, int 
*ptimeout)
                        if (!list_empty(&conn->out_list))
                                events |= POLLOUT;
                        conn->pollfd_idx = set_fd(conn->fd, events);
+                       /*
+                        * For stalled connection, we want to process the
+                        * pending command as soon as live-update has aborted.
+                        */
+                       if (conn->is_stalled && !lu_is_pending())
+                               *ptimeout = 0;
                }
        }
 }
@@ -1524,6 +1543,9 @@ static bool process_delayed_message(struct 
delayed_request *req)
        struct connection *conn = req->data;
        struct buffered_data *saved_in = conn->in;
 
+       if (lu_is_pending())
+               return false;
+
        /*
         * Part of process_message() expects conn->in to contains the
         * processed response. So save the current conn->in and restore it
@@ -1543,6 +1565,30 @@ static void consider_message(struct connection *conn)
                        sockmsg_string(conn->in->hdr.msg.type),
                        conn->in->hdr.msg.len, conn);
 
+       conn->is_stalled = false;
+       /*
+        * Currently, Live-Update is not supported if there is active
+        * transactions. In order to reduce the number of retry, delay
+        * any new request to start a transaction if Live-Update is pending
+        * and there are no transactions in-flight.
+        *
+        * If we can't delay the request, then mark the connection as
+        * stalled. This will ignore new requests until Live-Update happened
+        * or it was aborted.
+        */
+       if (lu_is_pending() && conn->transaction_started == 0 &&
+           conn->in->hdr.msg.type == XS_TRANSACTION_START) {
+               trace("Delaying transaction start for connection %p req_id 
%u\n",
+                     conn, conn->in->hdr.msg.req_id);
+
+               if (delay_request(conn, conn->in, process_delayed_message,
+                                 conn, false) != 0) {
+                       trace("Stalling connection %p\n", conn);
+                       conn->is_stalled = true;
+               }
+               return;
+       }
+
        process_message(conn, conn->in);
 
        assert(conn->in == NULL);
@@ -1629,6 +1675,7 @@ struct connection *new_connection(const struct 
interface_funcs *funcs)
        new->pollfd_idx = -1;
        new->funcs = funcs;
        new->is_ignored = false;
+       new->is_stalled = false;
        new->transaction_started = 0;
        INIT_LIST_HEAD(&new->out_list);
        INIT_LIST_HEAD(&new->watches);
diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index dac5171569..258f6ff382 100644
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -110,6 +110,9 @@ struct connection
        /* Is this connection ignored? */
        bool is_ignored;
 
+       /* Is the connection stalled? */
+       bool is_stalled;
+
        /* Buffered incoming data. */
        struct buffered_data *in;
 
--
generated by git-patchbot for /home/xen/git/xen.git#master