[Xen-devel] [PATCH 16/28] libxl: ao: Provide manip_refcnt

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Previously we used in_initiator to stop the ao being freed while we
were still in the initiator function (which would result in the
initiator's call to lixl__ao_inprogress accessing the ao after it had
been freed).

We are going to introduce a new libxl entrypoint which finds, and
operates on, ongoing aos.  This function needs the same protection,
and might even end up running on the same ao multiple times
concurrently.

So do this with reference counting instead, with a new variable
ao->manip_refcnt.

We keep ao->in_initiator because that allows us to keep some useful
asserts about the sequencing of libxl__ao_inprogress, etc.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
v3: Add a missing space.
    Mention locking in the comment.
---
 tools/libxl/libxl_event.c    |   43 +++++++++++++++++++++++++++++++++---------
 tools/libxl/libxl_internal.h |    1 +
 2 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index c95db5b..fffadf3 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -33,6 +33,8 @@
 
 static libxl__ao *ao_nested_root(libxl__ao *ao);
 
+static void ao__check_destroy(libxl_ctx *ctx, libxl__ao *ao);
+
 
 /*
  * The counter osevent_in_hook is used to ensure that the application
@@ -1345,8 +1347,7 @@ static void egc_run_callbacks(libxl__egc *egc)
         ao->how.callback(CTX, ao->rc, ao->how.u.for_callback);
         CTX_LOCK;
         ao->notified = 1;
-        if (!ao->in_initiator)
-            libxl__ao__destroy(CTX, ao);
+        ao__check_destroy(CTX, ao);
         CTX_UNLOCK;
     }
 }
@@ -1727,6 +1728,33 @@ int libxl_event_wait(libxl_ctx *ctx, libxl_event 
**event_r,
  *                              - destroy the ao
  */
 
+
+/*
+ * A "manip" is a libxl public function manipulating this ao, which
+ * has a pointer to it.  We have to not destroy it while that's the
+ * case, obviously.  Callers must have the ctx locked, obviously.
+ */
+static void ao__manip_enter(libxl__ao *ao)
+{
+    assert(ao->manip_refcnt < INT_MAX);
+    ao->manip_refcnt++;
+}
+
+static void ao__manip_leave(libxl_ctx *ctx, libxl__ao *ao)
+{
+    assert(ao->manip_refcnt > 0);
+    ao->manip_refcnt--;
+    ao__check_destroy(ctx, ao);
+}
+
+static void ao__check_destroy(libxl_ctx *ctx, libxl__ao *ao)
+{
+    if (!ao->manip_refcnt && ao->notified) {
+        assert(ao->complete);
+        libxl__ao__destroy(ctx, ao);
+    }
+}
+
 void libxl__ao__destroy(libxl_ctx *ctx, libxl__ao *ao)
 {
     AO_GC;
@@ -1808,8 +1836,8 @@ void libxl__ao_complete_check_progress_reports(libxl__egc 
*egc, libxl__ao *ao)
         }
         ao->notified = 1;
     }
-    if (!ao->in_initiator && ao->notified)
-        libxl__ao__destroy(ctx, ao);
+    
+    ao__check_destroy(ctx, ao);
 }
 
 libxl__ao *libxl__ao_create(libxl_ctx *ctx, uint32_t domid,
@@ -1824,6 +1852,7 @@ libxl__ao *libxl__ao_create(libxl_ctx *ctx, uint32_t 
domid,
     ao->magic = LIBXL__AO_MAGIC;
     ao->constructing = 1;
     ao->in_initiator = 1;
+    ao__manip_enter(ao);
     ao->poller = 0;
     ao->domid = domid;
     LIBXL_INIT_GC(ao->gc, ctx);
@@ -1904,11 +1933,7 @@ int libxl__ao_inprogress(libxl__ao *ao,
     }
 
     ao->in_initiator = 0;
-
-    if (ao->notified) {
-        assert(ao->complete);
-        libxl__ao__destroy(CTX,ao);
-    }
+    ao__manip_leave(CTX, ao);
 
     return rc;
 }
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index e29db43..d2c2637 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -449,6 +449,7 @@ struct libxl__ao {
      */
     uint32_t magic;
     unsigned constructing:1, in_initiator:1, complete:1, notified:1;
+    int manip_refcnt;
     libxl__ao *nested_root;
     int nested_progeny;
     int progress_reports_outstanding;
-- 
1.7.10.4


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