linux-kernel - Re: [PATCH v2] usb: dwc3: gadget: Replace list_for_each_entry

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Date:   Tue, 11 May 2021 11:13:48 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Wesley Cheng <wcheng@...eaurora.org>, gregkh@...uxfoundation.org,
        peter.chen@...nel.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        jackp@...eaurora.org, Wesley Cheng <wcheng@...eaurora.org>
Subject: Re: [PATCH v2] usb: dwc3: gadget: Replace
 list_for_each_entry_safe() if using giveback


Hi,

Wesley Cheng <wcheng@...eaurora.org> writes:
> 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 <wcheng@...eaurora.org>
> Reviewed-by: Peter Chen <peter.chen@...nel.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);

couldn't this be solved list_replace_init() instead? Then we can keep
using the regular list_for_each_entry_safe() which has an added semantic
meaning due to its name.

-- 
balbi

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