3.16.81-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Philip Oberstaller Philip.Oberstaller@septentrio.com
commit 3e9d3d2efc677b501b12512cab5adb4f32a0673a upstream.
When a single thread is sending out data over the gadget serial port, gs_start_tx() will be called both from the sender context and from the write completion. Since the port lock is released before the packet is queued, the order in which the URBs are submitted is not guaranteed. E.g.
sending thread completion (interrupt)
gs_write() LOCK gs_write_complete() LOCK (wait) gs_start_tx() req1 = list_entry(pool->next) UNLOCK LOCK (acquired) gs_start_tx() req2 = list_entry(pool->next) UNLOCK usb_ep_queue(req2) usb_ep_queue(req1)
I.e., req2 is submitted before req1 but it contains the data that comes after req1.
To reproduce, use SMP with sending thread and completion pinned to different CPUs, or use PREEMPT_RT, and add the following delay just before the call to usb_ep_queue():
if (port->write_started > 0 && !list_empty(pool)) udelay(1000);
To work around this problem, make sure that only one thread is running through the gs_start_tx() loop with an extra flag write_busy. Since gs_start_tx() is always called with the port lock held, no further synchronisation is needed. The original caller will continue through the loop when the request was successfully submitted.
Signed-off-by: Philip Oberstaller Philip.Oberstaller@septentrio.com Signed-off-by: Arnout Vandecappelle (Essensium/Mind) arnout@mind.be Signed-off-by: Felipe Balbi balbi@ti.com [bwh: Backported to 3.16: adjust filename] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/usb/gadget/u_serial.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
--- a/drivers/usb/gadget/u_serial.c +++ b/drivers/usb/gadget/u_serial.c @@ -116,6 +116,7 @@ struct gs_port { int write_allocated; struct gs_buf port_write_buf; wait_queue_head_t drain_wait; /* wait while writes drain */ + bool write_busy;
/* REVISIT this state ... */ struct usb_cdc_line_coding port_line_coding; /* 8-N-1 etc */ @@ -366,7 +367,7 @@ __acquires(&port->port_lock) int status = 0; bool do_tty_wake = false;
- while (!list_empty(pool)) { + while (!port->write_busy && !list_empty(pool)) { struct usb_request *req; int len;
@@ -396,9 +397,11 @@ __acquires(&port->port_lock) * NOTE that we may keep sending data for a while after * the TTY closed (dev->ioport->port_tty is NULL). */ + port->write_busy = true; spin_unlock(&port->port_lock); status = usb_ep_queue(in, req, GFP_ATOMIC); spin_lock(&port->port_lock); + port->write_busy = false;
if (status) { pr_debug("%s: %s %s err %d\n",