Re: [PATCH] usb: dwc3: gadget: Use list_replace_init() before traversing lists

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

Fix this condition by utilizing list_replace_init(), and traversing
through a local copy of the current elements in the endpoint lists.
This will also set the parent list as empty, so if another thread is
also looping through the list, it will be empty on the next iteration.

Fixes: d4f1afe5e896 ("usb: dwc3: gadget: move requests to cancelled_list")
Signed-off-by: Wesley Cheng <redacted>

---
Previous patchset:
https://lore.kernel.org/linux-usb/1620716636-12422-1-git-send-email-wcheng@codeaurora.org/ (local)
---
 drivers/usb/dwc3/gadget.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index a29a4ca..3ce6ed9 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c

@@ -1926,9 +1926,13 @@ static void dwc3_gadget_ep_cleanup_cancelled_requests(struct dwc3_ep *dep)
 {
 	struct dwc3_request		*req;
 	struct dwc3_request		*tmp;
+	struct list_head		local;
 	struct dwc3			*dwc = dep->dwc;
 
-	list_for_each_entry_safe(req, tmp, &dep->cancelled_list, list) {
+restart:
+	list_replace_init(&dep->cancelled_list, &local);