[PATCH 3.16 152/370] xhci: Handle command completion and timeout race

[Ben Hutchings]

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>

commit a5a1b9514154437aa1ed35c291191f82fd3e941a upstream.

If we get a command completion event at the same time as the command
timeout work starts on another cpu we might end up aborting the wrong
command.

If the command completion takes the xhci lock before the timeout work, it
will handle the command, pick the next command, mark it as current_cmd, and
re-queue the timeout work. When the timeout work finally gets the lock
It will start aborting the wrong command.

This case can be resolved by checking if the timeout work is pending inside
the timeout function itself. A new timeout work can only be pending if the
command completed and a new command was queued.

If there are no more commands pending then command completion will set
the current_cmd to NULL, which is already handled in the timeout work.

Reported-by: Baolin Wang <baolin.wang@xxxxxxxxxx>
Signed-off-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/usb/host/xhci-ring.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -1294,7 +1294,11 @@ void xhci_handle_command_timeout(unsigne
 
 	spin_lock_irqsave(&xhci->lock, flags);
 
-	if (!xhci->current_cmd) {
+	/*
+	 * If timeout work is pending, or current_cmd is NULL, it means we
+	 * raced with command completion. Command is handled so just return.
+	 */
+	if (!xhci->current_cmd || timer_pending(&xhci->cmd_timer)) {
 		spin_unlock_irqrestore(&xhci->lock, flags);
 		return;
 	}