linux-kernel - Re: [PATCH 1/2] usb: gadget: f_tcm: Use auto-cleanup for usb

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Message-ID: <CAKzKK0oAAFRXBALv38yOFz+4cyaM5B6ui8+WRXeM-sG32rUfJw@mail.gmail.com>
Date: Mon, 3 Nov 2025 17:54:28 +0800
From: Kuen-Han Tsai <khtsai@...gle.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Thinh Nguyen <Thinh.Nguyen@...opsys.com>, linux-usb@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] usb: gadget: f_tcm: Use auto-cleanup for usb_request

Hi Greg,

Thanks for the detailed review.

On Thu, Oct 30, 2025 at 11:33 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Thu, Oct 30, 2025 at 11:14:19PM +0800, Kuen-Han Tsai wrote:
> > Refactor f_tcm.c to use auto-cleanup mechanism for usb_request
> > allocations in bot_prepare_reqs(), uasp_alloc_stream_res(), and
> > uasp_alloc_cmd().
>
> Using guards are great for new code, or for bug fixes, but please don't
> just refactor code to use them for the sake of using them.  It makes it
> hard to review and justify the churn, especially when there is almost no
> code savings here at all.

You're absolutely right. The benefit doesn't justify the churn.

>
> > The explicit nullification of fu->..._req and stream->..._req pointers
> > on error is no longer needed. This is safe because these pointers are
> > only updated after all allocations within the function have succeeded.
> > If an error occurs, the fu structure members retain their previous
> > value, and the existing cleanup functions like bot_cleanup_old_alt() and
> > uasp_cleanup_old_alt() already handle stale pointers in the fu
> > structure.
>
> This seems to imply this is really fragile, and tricky, and maybe not
> worth it?
>
> The comment you added kind of enforces that feeling:
>
> > +     fu->bot_req_in = no_free_ptr(bot_req_in);
> > +     fu->bot_req_out = no_free_ptr(bot_req_out);
> > +
> > +     /* This line is placed here because free_usb_request also frees its
> > +      * buffer, which in this case points to the static fu->bot_status.csw.
> > +      */
>
> Which is "this line"?
>
> > +     status_req->buf = &fu->bot_status.csw;
>
> This one?
>
> > +     status_req->length = US_BULK_CS_WRAP_LEN;
>
> Or that one?
>
> Using guards for buffers for other structures is rough, as you have seen
> here, I don't really see the benefit at all, do you?
>

My apologies about the noise. After introducing the auto-cleanup
helpers recently, I had misunderstood that they were a preferred
pattern to proactively refactor existing goto logic with.

I see your point clearly now that these types of guards are best for
new codes or bug fixes, not for refactoring-only changes where the
benefit is minimal.

I will drop this patch series.

Regards,
Kuen-Han