[PATCH 2/5] usb: gadget: f_midi: added spinlock on transmit function

["Felipe F. Tonello" <eu@felipetonello.com>]

Since f_midi_transmit is called by both ALSA and USB frameworks, it can
potentially cause a race condition between both calls. This is bad because the
way f_midi_transmit is implemented can't handle concurrent calls. This is due
to the fact that the usb request fifo looks for the next element and only if
it has data to process it enqueues the request, otherwise re-uses it. If both
(ALSA and USB) frameworks calls this function at the same time, the
kfifo_seek() will return the same usb_request, which will cause a race
condition.

To solve this problem a syncronization mechanism is necessary. In this case it
is used a spinlock since f_midi_transmit is also called by usb_request->complete
callback in interrupt context.

On benchmarks realized by me, spinlocks were more efficient then scheduling
the f_midi_transmit tasklet in process context and using a mutex to
synchronize. Also it performs better then previous implementation that
allocated a usb_request for every new transmit made.

Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
---
 drivers/usb/gadget/function/f_midi.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
index 3cdb0741f3f8..8475e3dc82d4 100644
--- a/drivers/usb/gadget/function/f_midi.c
+++ b/drivers/usb/gadget/function/f_midi.c
@@ -24,6 +24,7 @@
 #include <linux/slab.h>
 #include <linux/device.h>
 #include <linux/kfifo.h>
+#include <linux/spinlock.h>
 
 #include <sound/core.h>
 #include <sound/initval.h>
@@ -95,6 +96,7 @@ struct f_midi {
 	unsigned int buflen, qlen;
 	/* This fifo is used as a buffer ring for pre-allocated IN usb_requests */
 	DECLARE_KFIFO_PTR(in_req_fifo, struct usb_request *);
+	spinlock_t transmit_lock;
 	unsigned int in_last_port;
 
 	struct gmidi_in_port	in_ports_array[/* in_ports */];
@@ -651,17 +653,22 @@ static void f_midi_transmit(struct f_midi *midi)
 {
 	struct usb_ep *ep = midi->in_ep;
 	int ret;
+	unsigned long flags;
 
 	/* We only care about USB requests if IN endpoint is enabled */
 	if (!ep || !ep->enabled)
 		goto drop_out;
 
+	spin_lock_irqsave(&midi->transmit_lock, flags);
+
 	do {
 		ret = f_midi_do_transmit(midi, ep);
 		if (ret < 0)
 			goto drop_out;
 	} while (ret);
 
+	spin_unlock_irqrestore(&midi->transmit_lock, flags);
+
 	return;
 
 drop_out:
@@ -1255,6 +1262,8 @@ static struct usb_function *f_midi_alloc(struct usb_function_instance *fi)
 	if (status)
 		goto setup_fail;
 
+	spin_lock_init(&midi->transmit_lock);
+
 	++opts->refcnt;
 	mutex_unlock(&opts->lock);
 
-- 
2.7.2