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Subject[PATCH v6 7/7] usb: gadget: u_serial: use mutex for serialising open()s
Remove home-made waiting mechanism from gs_open() and rely on
portmaster's mutex to do the job.

Note: This releases thread waiting on close() when another thread
open()s simultaneously.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>

---
v6: initial revision, new in the patchset

---
drivers/usb/gadget/function/u_serial.c | 112 ++++++++-----------------
1 file changed, 37 insertions(+), 75 deletions(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index a248ed0fd5d2..f986e5c55974 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -104,7 +104,6 @@ struct gs_port {
struct gs_console *console;
#endif

- bool openclose; /* open/close in progress */
u8 port_num;

struct list_head read_pool;
@@ -588,82 +587,45 @@ static int gs_open(struct tty_struct *tty, struct file *file)
{
int port_num = tty->index;
struct gs_port *port;
- int status;
+ int status = 0;

- do {
- mutex_lock(&ports[port_num].lock);
- port = ports[port_num].port;
- if (!port)
- status = -ENODEV;
- else {
- spin_lock_irq(&port->port_lock);
+ mutex_lock(&ports[port_num].lock);
+ port = ports[port_num].port;
+ if (!port) {
+ status = -ENODEV;
+ goto out;
+ }

- /* already open? Great. */
- if (port->port.count) {
- status = 0;
- port->port.count++;
-
- /* currently opening/closing? wait ... */
- } else if (port->openclose) {
- status = -EBUSY;
-
- /* ... else we do the work */
- } else {
- status = -EAGAIN;
- port->openclose = true;
- }
- spin_unlock_irq(&port->port_lock);
- }
- mutex_unlock(&ports[port_num].lock);
-
- switch (status) {
- default:
- /* fully handled */
- return status;
- case -EAGAIN:
- /* must do the work */
- break;
- case -EBUSY:
- /* wait for EAGAIN task to finish */
- msleep(1);
- /* REVISIT could have a waitchannel here, if
- * concurrent open performance is important
- */
- break;
- }
- } while (status != -EAGAIN);
-
- /* Do the "real open" */
spin_lock_irq(&port->port_lock);

/* allocate circular buffer on first open */
if (!kfifo_initialized(&port->port_write_buf)) {

spin_unlock_irq(&port->port_lock);
+
+ /*
+ * portmaster's mutex still protects from simultaneous open(),
+ * and close() can't happen, yet.
+ */
+
status = kfifo_alloc(&port->port_write_buf,
WRITE_BUF_SIZE, GFP_KERNEL);
- spin_lock_irq(&port->port_lock);
-
if (status) {
pr_debug("gs_open: ttyGS%d (%p,%p) no buffer\n",
- port->port_num, tty, file);
- port->openclose = false;
- goto exit_unlock_port;
+ port_num, tty, file);
+ goto out;
}
+
+ spin_lock_irq(&port->port_lock);
}

- /* REVISIT if REMOVED (ports[].port NULL), abort the open
- * to let rmmod work faster (but this way isn't wrong).
- */
-
- /* REVISIT maybe wait for "carrier detect" */
+ /* already open? Great. */
+ if (port->port.count++)
+ goto exit_unlock_port;

tty->driver_data = port;
port->port.tty = tty;

- port->port.count = 1;
- port->openclose = false;
-
/* if connected, start the I/O stream */
if (port->port_usb) {
struct gserial *gser = port->port_usb;
@@ -677,20 +639,21 @@ static int gs_open(struct tty_struct *tty, struct file *file)

pr_debug("gs_open: ttyGS%d (%p,%p)\n", port->port_num, tty, file);

- status = 0;
-
exit_unlock_port:
spin_unlock_irq(&port->port_lock);
+out:
+ mutex_unlock(&ports[port_num].lock);
return status;
}

-static int gs_writes_finished(struct gs_port *p)
+static int gs_close_flush_done(struct gs_port *p)
{
int cond;

- /* return true on disconnect or empty buffer */
+ /* return true on disconnect or empty buffer or if raced with open() */
spin_lock_irq(&p->port_lock);
- cond = (p->port_usb == NULL) || !kfifo_len(&p->port_write_buf);
+ cond = p->port_usb == NULL || !kfifo_len(&p->port_write_buf) ||
+ p->port.count > 1;
spin_unlock_irq(&p->port_lock);

return cond;
@@ -704,6 +667,7 @@ static void gs_close(struct tty_struct *tty, struct file *file)
spin_lock_irq(&port->port_lock);

if (port->port.count != 1) {
+raced_with_open:
if (port->port.count == 0)
WARN_ON(1);
else
@@ -713,12 +677,6 @@ static void gs_close(struct tty_struct *tty, struct file *file)

pr_debug("gs_close: ttyGS%d (%p,%p) ...\n", port->port_num, tty, file);

- /* mark port as closing but in use; we can drop port lock
- * and sleep if necessary
- */
- port->openclose = true;
- port->port.count = 0;
-
gser = port->port_usb;
if (gser && gser->disconnect)
gser->disconnect(gser);
@@ -729,9 +687,13 @@ static void gs_close(struct tty_struct *tty, struct file *file)
if (kfifo_len(&port->port_write_buf) > 0 && gser) {
spin_unlock_irq(&port->port_lock);
wait_event_interruptible_timeout(port->drain_wait,
- gs_writes_finished(port),
+ gs_close_flush_done(port),
GS_CLOSE_TIMEOUT * HZ);
spin_lock_irq(&port->port_lock);
+
+ if (port->port.count != 1)
+ goto raced_with_open;
+
gser = port->port_usb;
}

@@ -744,10 +706,9 @@ static void gs_close(struct tty_struct *tty, struct file *file)
else
kfifo_reset(&port->port_write_buf);

+ port->port.count = 0;
port->port.tty = NULL;

- port->openclose = false;
-
pr_debug("gs_close: ttyGS%d (%p,%p) done!\n",
port->port_num, tty, file);

@@ -1207,8 +1168,9 @@ static int gs_closed(struct gs_port *port)
int cond;

spin_lock_irq(&port->port_lock);
- cond = (port->port.count == 0) && !port->openclose;
+ cond = port->port.count == 0;
spin_unlock_irq(&port->port_lock);
+
return cond;
}

@@ -1413,7 +1375,7 @@ void gserial_disconnect(struct gserial *gser)

port->port_usb = NULL;
gser->ioport = NULL;
- if (port->port.count > 0 || port->openclose) {
+ if (port->port.count > 0) {
wake_up_interruptible(&port->drain_wait);
if (port->port.tty)
tty_hangup(port->port.tty);
@@ -1426,7 +1388,7 @@ void gserial_disconnect(struct gserial *gser)

/* finally, free any unused/unusable I/O buffers */
spin_lock_irqsave(&port->port_lock, flags);
- if (port->port.count == 0 && !port->openclose)
+ if (port->port.count == 0)
kfifo_free(&port->port_write_buf);
gs_free_requests(gser->out, &port->read_pool, NULL);
gs_free_requests(gser->out, &port->read_queue, NULL);
--
2.20.1
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