libxl: Fix eventloop_iteration over-locking

eventloop_iteration's head comment says that it must be called with
the ctx locked exactly once, and this is indeed true, and it's done
correctly at both the call sites.

However, it takes out the lock an additional time itself.  This is
wrong because it prevents the unlocks around poll from being
effective.  This would mean that a multithreaded event-loop using
program might suffer from undesired blocking, as one thread trying to
enter libxl might end up stalled by another thread waiting for a slow
event.  So remove those two lock calls.

Also add a couple of comments documenting the locking behaviour of
libxl__ao_inprogress and libxl__egc_cleanup.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

2 files changed, 2 insertions, 6 deletions

diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index c89add8f2f..5ac6334c1a 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -1058,8 +1058,6 @@ static int eventloop_iteration(libxl__egc *egc, libxl__poller *poller) {
     int rc;
     struct timeval now;
     
-    CTX_LOCK;
-
     rc = libxl__gettimeofday(gc, &now);
     if (rc) goto out;
 
@@ -1102,8 +1100,6 @@ static int eventloop_iteration(libxl__egc *egc, libxl__poller *poller) {
     afterpoll_internal(egc, poller,
                        poller->fd_polls_allocd, poller->fd_polls, now);
 
-    CTX_UNLOCK;
-
     rc = 0;
  out:
     return rc;
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 5167b7187a..04f13f6893 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -1191,7 +1191,7 @@ libxl__device_model_version_running(libxl__gc *gc, uint32_t domid);
 _hidden void libxl__egc_cleanup(libxl__egc *egc);
   /* Frees memory allocated within this egc's gc, and and report all
    * occurred events via callback, if applicable.  May reenter the
-   * application; see restrictions above. */
+   * application; see restrictions above.  The ctx must be UNLOCKED. */
 
 /* convenience macros: */
 
@@ -1287,7 +1287,7 @@ _hidden void libxl__egc_cleanup(libxl__egc *egc);
  * libxl__ao_inprogress MUST be called with the ctx locked exactly once. */
 _hidden libxl__ao *libxl__ao_create(libxl_ctx*, uint32_t domid,
                                     const libxl_asyncop_how*);
-_hidden int libxl__ao_inprogress(libxl__ao *ao);
+_hidden int libxl__ao_inprogress(libxl__ao *ao); /* temporarily unlocks */
 _hidden void libxl__ao_abort(libxl__ao *ao);
 _hidden void libxl__ao_complete(libxl__egc *egc, libxl__ao *ao, int rc);