linux-kernel - Re: [PATCH v2] USB: HCD: Fix URB giveback issue in tasklet function

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Message-ID: <Yt6haeQBseJOEx1r@rowland.harvard.edu>
Date:   Mon, 25 Jul 2022 09:58:01 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Weitao Wang <WeitaoWang-oc@...oxin.com>
Cc:     gregkh@...uxfoundation.org, kishon@...com, dianders@...omium.org,
        s.shtylyov@....ru, mka@...omium.org, ming.lei@...onical.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        tonywwang@...oxin.com, weitaowang@...oxin.com
Subject: Re: [PATCH v2] USB: HCD: Fix URB giveback issue in tasklet function

On Mon, Jul 25, 2022 at 02:52:51PM +0800, Weitao Wang wrote:

This is basically okay.  Just a couple of small comments...

> Usb core introduce the mechanism of giveback of URB in tasklet context to
> reduce hardware interrupt handling time. On some test situation(such as
> FIO with 4KB block size), when tasklet callback function called to
> giveback URB, interrupt handler add URB node to the bh->head list also.
> If check bh->head list again after finish all URB giveback of local_list,
> then it may introduce a "dynamic balance" between giveback URB and add URB
> to bh->head list. This tasklet callback function may not exit for a long
> time, which will cause other tasklet function calls to be delayed. Some
> real-time applications(such as KB and Mouse) will see noticeable lag.
> 
> Fix this issue by taking new URBs giveback in next tasklet function call.
> Add a member high_prio for structure giveback_urb_bh and replace the local
> high_prio_bh variable with this structure member in usb_hcd_giveback_urb.

The patch description should do more than say what the new code _is_ -- 
we can see that easily enough by reading the patch.  The description 
should explain _why_ the code was changed.

> Fixes: 94dfd7edfd5c ("USB: HCD: support giveback of URB in tasklet context")
> Signed-off-by: Weitao Wang <WeitaoWang-oc@...oxin.com>
> ---
> v1->v2:
>  - Fix compile warning by remove label "restart".
>  - Modify the patch description info.
>  - Change structure member from hi_priority to high_prio.
> 
>  drivers/usb/core/hcd.c  | 25 ++++++++++++++-----------
>  include/linux/usb/hcd.h |  1 +
>  2 files changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
> index 06eea8848ccc..1feb9a604380 100644
> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1691,7 +1691,6 @@ static void usb_giveback_urb_bh(struct tasklet_struct *t)
>  
>  	spin_lock_irq(&bh->lock);
>  	bh->running = true;
> - restart:
>  	list_replace_init(&bh->head, &local_list);
>  	spin_unlock_irq(&bh->lock);
>  
> @@ -1705,10 +1704,16 @@ static void usb_giveback_urb_bh(struct tasklet_struct *t)
>  		bh->completing_ep = NULL;
>  	}
>  
> -	/* check if there are new URBs to giveback */
> +	/* giveback new URBs next time to prevent this function from
> +	 * not exiting for a long time.
> +	 */

Minor stylistic issue: The currently accepted format for multi-line 
comments is like this:

	/*
	 * Blah blah blah
	 * Blah blah blah
	 */

Alan Stern