[Xen-devel] [PATCH v3 4/9] libxl: Add optimisation to ev_lock

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

It will often be the case that the lock is free to grab. So we first
try to grab it before we have to fork. Even though in this case the
locks are grabbed in the wrong order in the lock hierarchy (ev_lock
should be outside of CTX_LOCK), it is fine to try without blocking. If
that failed, we will release CTX_LOCK and try to grab both lock again
in the right order.

That optimisation is only enabled in releases (debug=n) so the more
complicated code with fork is actually exercised.

Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
---

Notes:
    v2:
    - new patch

 tools/libxl/Makefile         |  3 +++
 tools/libxl/libxl_internal.c | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/tools/libxl/Makefile b/tools/libxl/Makefile
index 6fdcbbddd6b6..4587a6fc9c65 100644
--- a/tools/libxl/Makefile
+++ b/tools/libxl/Makefile
@@ -35,6 +35,9 @@ ifeq ($(CONFIG_LIBNL),y)
 CFLAGS_LIBXL += $(LIBNL3_CFLAGS)
 endif
 CFLAGS_LIBXL += -Wshadow
+ifeq ($(debug),y)
+CFLAGS_LIBXL += -DCONFIG_DEBUG
+endif
 
 LIBXL_LIBS-$(CONFIG_ARM) += -lfdt
 
diff --git a/tools/libxl/libxl_internal.c b/tools/libxl/libxl_internal.c
index 28a126ccc342..a7a4d546c47a 100644
--- a/tools/libxl/libxl_internal.c
+++ b/tools/libxl/libxl_internal.c
@@ -620,6 +620,25 @@ static void ev_lock_prepare_fork(libxl__egc *egc, 
libxl__ev_devlock *lock)
     }
     fd = lock->fd;
 
+    /* Enable this optimisation only in releases, so the fork code is
+     * exercised while libxl is built with debug=y. */
+#ifndef CONFIG_DEBUG
+    /*
+     * We try to grab the lock before forking as it is likely to be free.
+     * Even though we are supposed to CTX_UNLOCK before attempting to grab
+     * the ev_lock, it is fine to do a non-blocking request now with the
+     * CTX_LOCK held as if that fails we'll try again in a fork (CTX_UNLOCK
+     * will be called in libxl), that will avoid deadlocks.
+     */
+    int r = flock(fd, LOCK_EX | LOCK_NB);
+    if (!r) {
+        libxl_fd_set_cloexec(CTX, fd, 1);
+        /* We held a lock, no need to fork but we need to check it. */
+        ev_lock_child_callback(egc, &lock->child, 0, 0);
+        return;
+    }
+#endif
+
     pid = libxl__ev_child_fork(gc, &lock->child, ev_lock_child_callback);
     if (pid < 0)
         goto out;
-- 
Anthony PERARD


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