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

[John Stultz <john.stultz@linaro.org>]

On Thu, Jul 29, 2021 at 12:34 AM Wesley Cheng <wcheng@codeaurora.org> wrote:
>
> 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 <wcheng@codeaurora.org>
>
> ---
> Previous patchset:
> https://lore.kernel.org/linux-usb/1620716636-12422-1-git-send-email-wcheng@codeaurora.org/
> ---
>  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);
> +
> +       list_for_each_entry_safe(req, tmp, &local, list) {
>                 dwc3_gadget_ep_skip_trbs(dep, req);
>                 switch (req->status) {
>                 case DWC3_REQUEST_STATUS_DISCONNECTED:
> @@ -1946,6 +1950,9 @@ static void dwc3_gadget_ep_cleanup_cancelled_requests(struct dwc3_ep *dep)
>                         break;
>                 }
>         }
> +
> +       if (!list_empty(&dep->cancelled_list))
> +               goto restart;
>  }

So, I'm not sure yet, but the "break" cases in the
list_for_each_entry_safe seem suspicious to me.

It seems we've move the list to the local listhead, then as we process
the local listhead, we may hit the "break" case, which will stop
processing the list, and then we end up returning, losing the
unprocessed items on the local listhead.

I suspect we need to move them back to the started/cancelled_list, or
rework things so we don't hit the "break" cases and fully process the
local list before returning.

thanks
-john