[PATCH v2 3/7] usb: mtu3: fix kernel panic at qmu transfer done irq handler

[Chunfeng Yun <chunfeng.yun@mediatek.com>]

When handle qmu transfer irq, it will unlock @mtu->lock before give back
request, if another thread handle disconnect event at the same time, and
try to disable ep, it may lock @mtu->lock and free qmu ring, then qmu
irq hanlder may get a NULL gpd, avoid the KE by checking gpd's value before
handling it.

e.g.
qmu done irq on cpu0                 thread running on cpu1

qmu_done_tx()
  handle gpd [0]
    mtu3_requ_complete()        mtu3_gadget_ep_disable()
      unlock @mtu->lock
        give back request         lock @mtu->lock
                                    mtu3_ep_disable()
                                      mtu3_gpd_ring_free()
                                   unlock @mtu->lock
      lock @mtu->lock
    get next gpd [1]

[1]: goto [0] to handle next gpd, and next gpd may be NULL.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
---
v2: fix typo suggested by AngeloGioacchino
---
 drivers/usb/mtu3/mtu3_qmu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/mtu3/mtu3_qmu.c b/drivers/usb/mtu3/mtu3_qmu.c
index 6be4977a5db5..3d77408e3133 100644
--- a/drivers/usb/mtu3/mtu3_qmu.c
+++ b/drivers/usb/mtu3/mtu3_qmu.c
@@ -210,6 +210,7 @@ static struct qmu_gpd *advance_enq_gpd(struct mtu3_gpd_ring *ring)
 	return ring->enqueue;
 }
 
+/* @dequeue may be NULL if ring is unallocated or freed */
 static struct qmu_gpd *advance_deq_gpd(struct mtu3_gpd_ring *ring)
 {
 	if (ring->dequeue < ring->end)
@@ -522,7 +523,7 @@ static void qmu_done_tx(struct mtu3 *mtu, u8 epnum)
 	dev_dbg(mtu->dev, "%s EP%d, last=%p, current=%p, enq=%p\n",
 		__func__, epnum, gpd, gpd_current, ring->enqueue);
 
-	while (gpd != gpd_current && !GET_GPD_HWO(gpd)) {
+	while (gpd && gpd != gpd_current && !GET_GPD_HWO(gpd)) {
 
 		mreq = next_request(mep);
 
@@ -561,7 +562,7 @@ static void qmu_done_rx(struct mtu3 *mtu, u8 epnum)
 	dev_dbg(mtu->dev, "%s EP%d, last=%p, current=%p, enq=%p\n",
 		__func__, epnum, gpd, gpd_current, ring->enqueue);
 
-	while (gpd != gpd_current && !GET_GPD_HWO(gpd)) {
+	while (gpd && gpd != gpd_current && !GET_GPD_HWO(gpd)) {
 
 		mreq = next_request(mep);
 
-- 
2.18.0