[PATCH tty-next v2 3/4] serial: kgdb_nmi: Switch from tasklets to real timers

[Daniel Thompson]

kgdb_nmi uses tasklets on the assumption they will not be scheduled
until the next timer tick. This assumption is invalid and can lead to
live lock, continually servicing the kgdb_nmi tasklet. This is fixed
by using the timer API instead.

Signed-off-by: Daniel Thompson <daniel.thomp...@linaro.org>
---
 drivers/tty/serial/kgdb_nmi.c | 30 +++++++++---------------------
 1 file changed, 9 insertions(+), 21 deletions(-)

diff --git a/drivers/tty/serial/kgdb_nmi.c b/drivers/tty/serial/kgdb_nmi.c
index d51b2a1..20d21d0 100644
--- a/drivers/tty/serial/kgdb_nmi.c
+++ b/drivers/tty/serial/kgdb_nmi.c
@@ -80,24 +80,10 @@ static struct console kgdb_nmi_console = {
 
 struct kgdb_nmi_tty_priv {
        struct tty_port port;
-       struct tasklet_struct tlet;
+       struct timer_list timer;
        STRUCT_KFIFO(char, KGDB_NMI_FIFO_SIZE) fifo;
 };
 
-/*
- * Our debugging console is polled in a tasklet, so we'll check for input
- * every tick. In HZ-less mode, we should program the next tick.  We have
- * to use the lowlevel stuff as no locks should be grabbed.
- */
-#ifdef CONFIG_HIGH_RES_TIMERS
-static void kgdb_tty_poke(void)
-{
-       tick_program_event(ktime_get(), 0);
-}
-#else
-static inline void kgdb_tty_poke(void) {}
-#endif
-
 static struct tty_port *kgdb_nmi_port;
 
 static void kgdb_tty_recv(int ch)
@@ -108,14 +94,13 @@ static void kgdb_tty_recv(int ch)
        if (!kgdb_nmi_port || ch < 0)
                return;
        /*
-        * Can't use port->tty->driver_data as tty might be not there. Tasklet
+        * Can't use port->tty->driver_data as tty might be not there. Timer
         * will check for tty and will get the ref, but here we don't have to
         * do that, and actually, we can't: we're in NMI context, no locks are
         * possible.
         */
        priv = container_of(kgdb_nmi_port, struct kgdb_nmi_tty_priv, port);
        kfifo_in(&priv->fifo, &c, 1);
-       kgdb_tty_poke();
 }
 
 static int kgdb_nmi_poll_one_knock(void)
@@ -199,7 +184,8 @@ static void kgdb_nmi_tty_receiver(unsigned long data)
        struct kgdb_nmi_tty_priv *priv = (void *)data;
        char ch;
 
-       tasklet_schedule(&priv->tlet);
+       priv->timer.expires = jiffies + (HZ/100);
+       add_timer(&priv->timer);
 
        if (likely(!kgdb_nmi_tty_enabled || !kfifo_len(&priv->fifo)))
                return;
@@ -215,7 +201,9 @@ static int kgdb_nmi_tty_activate(struct tty_port *port, 
struct tty_struct *tty)
            container_of(port, struct kgdb_nmi_tty_priv, port);
 
        kgdb_nmi_port = port;
-       tasklet_schedule(&priv->tlet);
+       priv->timer.expires = jiffies + (HZ/100);
+       add_timer(&priv->timer);
+
        return 0;
 }
 
@@ -224,7 +212,7 @@ static void kgdb_nmi_tty_shutdown(struct tty_port *port)
        struct kgdb_nmi_tty_priv *priv =
            container_of(port, struct kgdb_nmi_tty_priv, port);
 
-       tasklet_kill(&priv->tlet);
+       del_timer(&priv->timer);
        kgdb_nmi_port = NULL;
 }
 
@@ -243,7 +231,7 @@ static int kgdb_nmi_tty_install(struct tty_driver *drv, 
struct tty_struct *tty)
                return -ENOMEM;
 
        INIT_KFIFO(priv->fifo);
-       tasklet_init(&priv->tlet, kgdb_nmi_tty_receiver, (unsigned long)priv);
+       setup_timer(&priv->timer, kgdb_nmi_tty_receiver, (unsigned long)priv);
        tty_port_init(&priv->port);
        priv->port.ops = &kgdb_nmi_tty_port_ops;
        tty->driver_data = priv;
-- 
1.9.0

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