[Xen-changelog] [xen-unstable] libxl: Fix eventloop_iteration over-locking

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
# Date 1334150049 -3600
# Node ID 081230edcb5e508e622bdd8b806862f76d452e2d
# Parent  352e75baadeac4edc0e062d41692f1d428827204
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@xxxxxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Committed-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
---


diff -r 352e75baadea -r 081230edcb5e tools/libxl/libxl_event.c
--- a/tools/libxl/libxl_event.c Wed Apr 11 14:14:09 2012 +0100
+++ b/tools/libxl/libxl_event.c Wed Apr 11 14:14:09 2012 +0100
@@ -1058,8 +1058,6 @@ static int eventloop_iteration(libxl__eg
     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__eg
     afterpoll_internal(egc, poller,
                        poller->fd_polls_allocd, poller->fd_polls, now);
 
-    CTX_UNLOCK;
-
     rc = 0;
  out:
     return rc;
diff -r 352e75baadea -r 081230edcb5e tools/libxl/libxl_internal.h
--- a/tools/libxl/libxl_internal.h      Wed Apr 11 14:14:09 2012 +0100
+++ b/tools/libxl/libxl_internal.h      Wed Apr 11 14:14:09 2012 +0100
@@ -1191,7 +1191,7 @@ libxl__device_model_version_running(libx
 _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__e
  * 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);
 

_______________________________________________
Xen-changelog mailing list
Xen-changelog@xxxxxxxxxxxxx
http://lists.xensource.com/xen-changelog