USB: cdc-acm: clean up throttle handling

Clean up the throttle implementation by dropping the redundant
throttle_req flag which was a remnant from back when USB serial had only
a single read URB, something which was later carried over to cdc-acm.

Also convert the throttled flag to an atomic bit flag.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -468,7 +468,6 @@ static void acm_read_bulk_callback(struct urb *urb)
 {
 	struct acm_rb *rb = urb->context;
 	struct acm *acm = rb->instance;
-	unsigned long flags;
 	int status = urb->status;
 	bool stopped = false;
 	bool stalled = false;
@@ -525,15 +524,10 @@ static void acm_read_bulk_callback(struct urb *urb)
 		return;
 	}
 
-	/* throttle device if requested by tty */
-	spin_lock_irqsave(&acm->read_lock, flags);
-	acm->throttled = acm->throttle_req;
-	if (!acm->throttled) {
-		spin_unlock_irqrestore(&acm->read_lock, flags);
-		acm_submit_read_urb(acm, rb->index, GFP_ATOMIC);
-	} else {
-		spin_unlock_irqrestore(&acm->read_lock, flags);
-	}
+	if (test_bit(ACM_THROTTLED, &acm->flags))
+		return;
+
+	acm_submit_read_urb(acm, rb->index, GFP_ATOMIC);
 }
 
 /* data interface wrote those outgoing bytes */
@@ -670,10 +664,7 @@ static int acm_port_activate(struct tty_port *port, struct tty_struct *tty)
 	/*
 	 * Unthrottle device in case the TTY was closed while throttled.
 	 */
-	spin_lock_irq(&acm->read_lock);
-	acm->throttled = 0;
-	acm->throttle_req = 0;
-	spin_unlock_irq(&acm->read_lock);
+	clear_bit(ACM_THROTTLED, &acm->flags);
 
 	retval = acm_submit_read_urbs(acm, GFP_KERNEL);
 	if (retval)
@@ -841,27 +832,19 @@ static void acm_tty_throttle(struct tty_struct *tty)
 {
 	struct acm *acm = tty->driver_data;
 
-	spin_lock_irq(&acm->read_lock);
-	acm->throttle_req = 1;
-	spin_unlock_irq(&acm->read_lock);
+	set_bit(ACM_THROTTLED, &acm->flags);
 }
 
 static void acm_tty_unthrottle(struct tty_struct *tty)
 {
 	struct acm *acm = tty->driver_data;
-	unsigned int was_throttled;
 
-	spin_lock_irq(&acm->read_lock);
-	was_throttled = acm->throttled;
-	acm->throttled = 0;
-	acm->throttle_req = 0;
-	spin_unlock_irq(&acm->read_lock);
+	clear_bit(ACM_THROTTLED, &acm->flags);
 
 	/* Matches the smp_mb__after_atomic() in acm_read_bulk_callback(). */
 	smp_mb();
 
-	if (was_throttled)
-		acm_submit_read_urbs(acm, GFP_KERNEL);
+	acm_submit_read_urbs(acm, GFP_KERNEL);
 }
 
 static int acm_tty_break_ctl(struct tty_struct *tty, int state)

drivers/usb/class/cdc-acm.h

@@ -108,6 +108,7 @@ struct acm {
 	unsigned long flags;
 #		define EVENT_TTY_WAKEUP	0
 #		define EVENT_RX_STALL	1
+#		define ACM_THROTTLED	2
 	struct usb_cdc_line_coding line;		/* bits, stop, parity */
 	struct work_struct work;			/* work queue entry for line discipline waking up */
 	unsigned int ctrlin;				/* input control lines (DCD, DSR, RI, break, overruns) */
@@ -122,8 +123,6 @@ struct acm {
 	unsigned int ctrl_caps;				/* control capabilities from the class specific header */
 	unsigned int susp_count;			/* number of suspended interfaces */
 	unsigned int combined_interfaces:1;		/* control and data collapsed */
-	unsigned int throttled:1;			/* actually throttled */
-	unsigned int throttle_req:1;			/* throttle requested */
 	u8 bInterval;
 	struct usb_anchor delayed;			/* writes queued for a device about to be woken */
 	unsigned long quirks;