Bump minimum kernel version to 2.6.30 to use EFD_SEMAPHORE

The bump in the kernel version allows the use of EFD_SEMAPHORE for
eventfd.

Adjust the README.md to reflect this. No need to provide direct
instruction for older kernel. We leave the '--disable-epoll' switch
available and the code behind it simply because other platform might not
have epoll available.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id512f018c5394c9cc699e19c3d5a0d753e56414b

README.adoc

@@ -52,11 +52,7 @@ endif::env-github[]
 You need the following dependencies to build and run the {lt}
 components:
 
-* **Linux kernel{nbsp}≥{nbsp}2.6.27**
-+
-Use `{nbh}{nbh}disable{nbh}epoll` at <<configure,build configuration>>
-time to build {lt} for an older kernel. However, note that we can't
-provide any guarantee below 2.6.27.
+* **Linux kernel{nbsp}≥{nbsp}2.6.30**
 
 * **http://www.liburcu.org/[Userspace{nbsp}RCU]{nbsp}≥{nbsp}0.11.0**.
 +
@@ -193,9 +189,6 @@ If you want to embed the manual pages into the `lttng`,
 so that you don't need `man` to view them, use the
 `{nbh}{nbh}enable{nbh}embedded{nbh}help` option.
 
-If your Linux kernel is older than 2.6.27, use the
-`{nbh}{nbh}disable{nbh}epoll` option.
-
 This build configuration script finds LTTng{nbh}UST with
 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg{nbh}config]:
 set the `PKG_CONFIG_PATH` environment variable accordingly if

src/bin/lttng-sessiond/notification-thread-events.cpp

@@ -3137,18 +3137,6 @@ int pop_cmd_queue(struct notification_thread_handle *handle,
 		goto error_unlock;
 	}
 
-	/* Simulate behaviour of EFD_SEMAPHORE for older kernels. */
-	counter -= 1;
-	if (counter != 0) {
-		ret = lttng_write(handle->cmd_queue.event_fd, &counter,
-				sizeof(counter));
-		if (ret != sizeof(counter)) {
-			PERROR("Failed to write back to event_fd for EFD_SEMAPHORE emulation");
-			ret = -1;
-			goto error_unlock;
-		}
-	}
-
 	*cmd = cds_list_first_entry(&handle->cmd_queue.list,
 			struct notification_thread_command, cmd_list_node);
 	cds_list_del(&((*cmd)->cmd_list_node));

src/bin/lttng-sessiond/notification-thread.cpp

@@ -105,7 +105,7 @@ struct notification_thread_handle *notification_thread_handle_create(
 
 	sem_init(&handle->ready, 0, 0);
 
-	event_fd =  eventfd(0, EFD_CLOEXEC);
+	event_fd =  eventfd(0, EFD_CLOEXEC | EFD_SEMAPHORE);
 	if (event_fd < 0) {
 		PERROR("event_fd creation");
 		goto error;