[PATCH] evdev: discard oldest event on buffer overflow
From: Bill Gatliff <hidden>
Date: 2010-10-27 17:10:11
Subsystem:
input (keyboard, mouse, joystick, touchscreen) drivers, the rest · Maintainers:
Dmitry Torokhov, Linus Torvalds
On a buffer overflow, the head and tail pointers collide. This makes the buffer suddenly look empty, rather than full, which may confuse applications. This patch moves the tail pointer up one event on a buffer overflow, which "consumes" the oldest event in the buffer to make room for the incoming event. Thus, although data is lost due to the overflow (which is unavoidable), the data that remains is both recent and still time-ordered. Signed-off-by: Bill Gatliff <redacted> --- drivers/input/evdev.c | 18 +++++++++++------- 1 files changed, 11 insertions(+), 7 deletions(-)
diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
index e3f7fc6..04a18ff 100644
--- a/drivers/input/evdev.c
+++ b/drivers/input/evdev.c@@ -55,15 +55,19 @@ static void evdev_pass_event(struct evdev_client *client, { /* * Interrupts are disabled, just acquire the lock. - * Make sure we don't leave with the client buffer - * "empty" by having client->head == client->tail. */ spin_lock(&client->buffer_lock); - do { - client->buffer[client->head++] = *event; - client->head &= client->bufsize - 1; - } while (client->head == client->tail); - spin_unlock(&client->buffer_lock); + client->buffer[client->head++] = *event; + client->head &= client->bufsize - 1; + + /* + * If we overflow the buffer, consume the oldest + * event to make room for the one that just arrived + */ + if (unlikely(client->head == client->tail)) { + client->tail++; + client->tail &= client->bufsize - 1; + } if (event->type == EV_SYN) kill_fasync(&client->fasync, SIGIO, POLL_IN);
--
1.7.1