9pfs/xen: Fix segfault on shutdown

xen_9pfs_free can't use gnttabdev since it is already closed and NULL-ed
out when free is called.  Do the teardown in _disconnect().  This
matches the setup done in _connect().

trace-events are also added for the XenDevOps functions.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Message-Id: <20230502143722.15613-1-jandryuk@gmail.com>
[C.S.: - Remove redundant return in xen_9pfs_free().
       - Add comment to trace-events. ]
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
(cherry picked from commit 92e667f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/9pfs/trace-events

@@ -48,3 +48,9 @@ v9fs_readlink(uint16_t tag, uint8_t id, int32_t fid) "tag %d id %d fid %d"
 v9fs_readlink_return(uint16_t tag, uint8_t id, char* target) "tag %d id %d name %s"
 v9fs_setattr(uint16_t tag, uint8_t id, int32_t fid, int32_t valid, int32_t mode, int32_t uid, int32_t gid, int64_t size, int64_t atime_sec, int64_t mtime_sec) "tag %u id %u fid %d iattr={valid %d mode %d uid %d gid %d size %"PRId64" atime=%"PRId64" mtime=%"PRId64" }"
 v9fs_setattr_return(uint16_t tag, uint8_t id) "tag %u id %u"
+
+# xen-9p-backend.c
+xen_9pfs_alloc(char *name) "name %s"
+xen_9pfs_connect(char *name) "name %s"
+xen_9pfs_disconnect(char *name) "name %s"
+xen_9pfs_free(char *name) "name %s"

hw/9pfs/xen-9p-backend.c

@@ -25,6 +25,8 @@
 #include "qemu/iov.h"
 #include "fsdev/qemu-fsdev.h"
 
+#include "trace.h"
+
 #define VERSIONS "1"
 #define MAX_RINGS 8
 #define MAX_RING_ORDER 9
@@ -336,6 +338,8 @@ static void xen_9pfs_disconnect(struct XenLegacyDevice *xendev)
     Xen9pfsDev *xen_9pdev = container_of(xendev, Xen9pfsDev, xendev);
     int i;
 
+    trace_xen_9pfs_disconnect(xendev->name);
+
     for (i = 0; i < xen_9pdev->num_rings; i++) {
         if (xen_9pdev->rings[i].evtchndev != NULL) {
             qemu_set_fd_handler(qemu_xen_evtchn_fd(xen_9pdev->rings[i].evtchndev),
@@ -344,40 +348,41 @@ static void xen_9pfs_disconnect(struct XenLegacyDevice *xendev)
                                    xen_9pdev->rings[i].local_port);
             xen_9pdev->rings[i].evtchndev = NULL;
         }
-    }
-}
-
-static int xen_9pfs_free(struct XenLegacyDevice *xendev)
-{
-    Xen9pfsDev *xen_9pdev = container_of(xendev, Xen9pfsDev, xendev);
-    int i;
-
-    if (xen_9pdev->rings[0].evtchndev != NULL) {
-        xen_9pfs_disconnect(xendev);
-    }
-
-    for (i = 0; i < xen_9pdev->num_rings; i++) {
         if (xen_9pdev->rings[i].data != NULL) {
             xen_be_unmap_grant_refs(&xen_9pdev->xendev,
                                     xen_9pdev->rings[i].data,
                                     xen_9pdev->rings[i].intf->ref,
                                     (1 << xen_9pdev->rings[i].ring_order));
+            xen_9pdev->rings[i].data = NULL;
         }
         if (xen_9pdev->rings[i].intf != NULL) {
             xen_be_unmap_grant_ref(&xen_9pdev->xendev,
                                    xen_9pdev->rings[i].intf,
                                    xen_9pdev->rings[i].ref);
+            xen_9pdev->rings[i].intf = NULL;
         }
         if (xen_9pdev->rings[i].bh != NULL) {
             qemu_bh_delete(xen_9pdev->rings[i].bh);
+            xen_9pdev->rings[i].bh = NULL;
         }
     }
 
     g_free(xen_9pdev->id);
+    xen_9pdev->id = NULL;
     g_free(xen_9pdev->tag);
+    xen_9pdev->tag = NULL;
     g_free(xen_9pdev->path);
+    xen_9pdev->path = NULL;
     g_free(xen_9pdev->security_model);
+    xen_9pdev->security_model = NULL;
     g_free(xen_9pdev->rings);
+    xen_9pdev->rings = NULL;
+}
+
+static int xen_9pfs_free(struct XenLegacyDevice *xendev)
+{
+    trace_xen_9pfs_free(xendev->name);
+
     return 0;
 }
 
@@ -389,6 +394,8 @@ static int xen_9pfs_connect(struct XenLegacyDevice *xendev)
     V9fsState *s = &xen_9pdev->state;
     QemuOpts *fsdev;
 
+    trace_xen_9pfs_connect(xendev->name);
+
     if (xenstore_read_fe_int(&xen_9pdev->xendev, "num-rings",
                              &xen_9pdev->num_rings) == -1 ||
         xen_9pdev->num_rings > MAX_RINGS || xen_9pdev->num_rings < 1) {
@@ -496,6 +503,8 @@ static int xen_9pfs_connect(struct XenLegacyDevice *xendev)
 
 static void xen_9pfs_alloc(struct XenLegacyDevice *xendev)
 {
+    trace_xen_9pfs_alloc(xendev->name);
+
     xenstore_write_be_str(xendev, "versions", VERSIONS);
     xenstore_write_be_int(xendev, "max-rings", MAX_RINGS);
     xenstore_write_be_int(xendev, "max-ring-page-order", MAX_RING_ORDER);