[Xen-devel] [PATCH 2/6] libxl: events: Deregister xenstore watch fd when not needed

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

We want to have no fd events registered when we are idle.
In this patch, deal with the xenstore watch fd:

* Track the total number of active watches.
* When deregistering a watch, or when watch registration fails, check
  whether there are now no watches and if so deregister the fd.
* On libxl teardown, the watch fd should therefore be unregistered.
  assert that this is the case.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---
 tools/libxl/libxl.c          |    2 +-
 tools/libxl/libxl_event.c    |   11 +++++++++++
 tools/libxl/libxl_internal.h |    2 +-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index c111f27..12a1013 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -164,7 +164,7 @@ int libxl_ctx_free(libxl_ctx *ctx)
 
     for (i = 0; i < ctx->watch_nslots; i++)
         assert(!libxl__watch_slot_contents(gc, i));
-    libxl__ev_fd_deregister(gc, &ctx->watch_efd);
+    assert(!libxl__ev_fd_isregistered(&ctx->watch_efd));
     libxl__ev_fd_deregister(gc, &ctx->evtchn_efd);
     libxl__ev_fd_deregister(gc, &ctx->sigchld_selfpipe_efd);
 
diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index 22b1227..da0a20e 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -524,6 +524,13 @@ static char *watch_token(libxl__gc *gc, int slotnum, 
uint32_t counterval)
     return libxl__sprintf(gc, "%d/%"PRIx32, slotnum, counterval);
 }
 
+static void watches_check_fd_deregister(libxl__gc *gc)
+{
+    assert(CTX->nwatches>=0);
+    if (!CTX->nwatches)
+        libxl__ev_fd_deregister(gc, &CTX->watch_efd);
+}
+
 int libxl__ev_xswatch_register(libxl__gc *gc, libxl__ev_xswatch *w,
                                libxl__ev_xswatch_callback *func,
                                const char *path /* copied */)
@@ -579,6 +586,7 @@ int libxl__ev_xswatch_register(libxl__gc *gc, 
libxl__ev_xswatch *w,
     w->slotnum = slotnum;
     w->path = path_copy;
     w->callback = func;
+    CTX->nwatches++;
     libxl__set_watch_slot_contents(use, w);
 
     CTX_UNLOCK;
@@ -590,6 +598,7 @@ int libxl__ev_xswatch_register(libxl__gc *gc, 
libxl__ev_xswatch *w,
     if (use)
         LIBXL_SLIST_INSERT_HEAD(&CTX->watch_freeslots, use, empty);
     free(path_copy);
+    watches_check_fd_deregister(gc);
     CTX_UNLOCK;
     return rc;
 }
@@ -614,6 +623,8 @@ void libxl__ev_xswatch_deregister(libxl__gc *gc, 
libxl__ev_xswatch *w)
         libxl__ev_watch_slot *slot = &CTX->watch_slots[w->slotnum];
         LIBXL_SLIST_INSERT_HEAD(&CTX->watch_freeslots, slot, empty);
         w->slotnum = -1;
+        CTX->nwatches--;
+        watches_check_fd_deregister(gc);
     } else {
         LOG(DEBUG, "watch w=%p: deregister unregistered", w);
     }
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 4361421..728fe2c 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -352,7 +352,7 @@ struct libxl__ctx {
     LIBXL_TAILQ_HEAD(, libxl__ev_time) etimes;
 
     libxl__ev_watch_slot *watch_slots;
-    int watch_nslots;
+    int watch_nslots, nwatches;
     LIBXL_SLIST_HEAD(, libxl__ev_watch_slot) watch_freeslots;
     uint32_t watch_counter; /* helps disambiguate slot reuse */
     libxl__ev_fd watch_efd;
-- 
1.7.10.4


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