[Xen-changelog] [xen staging] libxl: Add optimisation to ev_lock

[patchbot@xxxxxxx]

commit 5e2d477993fd9f759b87ed6391be52a3638a02dc
Author:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
AuthorDate: Fri Jun 7 15:19:02 2019 +0100
Commit:     Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
CommitDate: Thu Sep 19 12:24:56 2019 +0100

    libxl: Add optimisation to ev_lock
    
    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>
---
 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 6fdcbbddd6..4587a6fc9c 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 28a126ccc3..a7a4d546c4 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;
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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