Re: [PATCH v2] usb: dwc3: gadget: Replace list_for_each_entry_safe() if using giveback

The list_for_each_entry_safe() macro saves the current item (n) and
the item after (n+1), so that n can be safely removed without
corrupting the list.  However, when traversing the list and removing
items using gadget giveback, the DWC3 lock is briefly released,
allowing other routines to execute.  There is a situation where, while
items are being removed from the cancelled_list using
dwc3_gadget_ep_cleanup_cancelled_requests(), the pullup disable
routine is running in parallel (due to UDC unbind).  As the cleanup
routine removes n, and the pullup disable removes n+1, once the
cleanup retakes the DWC3 lock, it references a request who was already
removed/handled.  With list debug enabled, this leads to a panic.
Ensure all instances of the macro are replaced where gadget giveback
is used.

Example call stack:

Thread#1:
__dwc3_gadget_ep_set_halt() - CLEAR HALT
  -> dwc3_gadget_ep_cleanup_cancelled_requests()
    ->list_for_each_entry_safe()
    ->dwc3_gadget_giveback(n)
      ->dwc3_gadget_del_and_unmap_request()- n deleted[cancelled_list]
      ->spin_unlock
      ->Thread#2 executes
      ...
    ->dwc3_gadget_giveback(n+1)
      ->Already removed!

Thread#2:
dwc3_gadget_pullup()
  ->waiting for dwc3 spin_lock
  ...
  ->Thread#1 released lock
  ->dwc3_stop_active_transfers()
    ->dwc3_remove_requests()
      ->fetches n+1 item from cancelled_list (n removed by Thread#1)
      ->dwc3_gadget_giveback()
        ->dwc3_gadget_del_and_unmap_request()- n+1 deleted[cancelled_list]
        ->spin_unlock

Fixes: d4f1afe5e896 ("usb: dwc3: gadget: move requests to cancelled_list")
Signed-off-by: Wesley Cheng <redacted>
Reviewed-by: Peter Chen <peter.chen@kernel.org>
---
Changes in v2:
 - Updated commit message with context call stack of an example scenario
   seen on device.

 drivers/usb/dwc3/gadget.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index dd80e5c..efa939b 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c

@@ -1737,10 +1737,10 @@ static void dwc3_gadget_ep_skip_trbs(struct dwc3_ep *dep, struct dwc3_request *r
 static void dwc3_gadget_ep_cleanup_cancelled_requests(struct dwc3_ep *dep)
 {
 	struct dwc3_request		*req;
-	struct dwc3_request		*tmp;
 	struct dwc3			*dwc = dep->dwc;
 
-	list_for_each_entry_safe(req, tmp, &dep->cancelled_list, list) {
+	while (!list_empty(&dep->cancelled_list)) {
+		req = next_request(&dep->cancelled_list);