linux-kernel - Re: [PATCH v4] kcov, usb: only collect coverage from __usb_hcd_giveback

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Message-ID: <CAAeHK+wC4uHz0-g8jEp=Nn11De1EpDsNeOLbXxrR2hJ3-2DFkA@mail.gmail.com>
Date:   Tue, 24 Nov 2020 00:42:27 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Shuah Khan <shuah@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Aleksandr Nogikh <nogikh@...gle.com>,
        Nazime Hande Harputluoglu <handeharput@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] kcov, usb: only collect coverage from
 __usb_hcd_giveback_urb in softirq

On Fri, Nov 13, 2020 at 4:47 PM Marco Elver <elver@...gle.com> wrote:
>
> On Fri, 13 Nov 2020 at 14:42, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> > On Fri, Nov 13, 2020 at 2:28 PM Sebastian Andrzej Siewior
> > <bigeasy@...utronix.de> wrote:
> > >
> > > On 2020-11-13 13:51:19 [+0100], Andrey Konovalov wrote:
> > > > Hi Sebastian,
> > >
> > > Hi Andrey,
> > >
> > > > Replaced with what and why?
> > >
> > > Linus requested in
> > >         https://lkml.kernel.org/r/CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@mail.gmail.com/
> > >
> > > that drivers should not change their behaviour on context magic like
> > > in_atomic(), in_interrupt() and so on.
> > > The USB bits were posted in
> > >         https://lkml.kernel.org/r/20201019100629.419020859@linutronix.de
>
> Arguably this patch is *not* changing "driver behaviour", it's only
> changing how and when KCOV collects coverage, which is not related to
> how the driver behaves.
>
> > > and merged (which is probably the same time as this patch).
> > >
> > > I haven't look what this code should do or does but there are HCDs for
> > > which this is never true like the UHCI/OHCI controller for instance.
> >
> > We could go back to adding softirq-specific kcov callbacks. Perhaps
> > with a simpler implementation than what we had before to only cover
> > this case. Something like kcov_remote_start_usb_softirq() and
> > kcov_remote_stop_softirq() that do the softirq check internally.
>
> Is this a matter of simply banning such functions entirely without
> understanding their use? Because that sounds wrong. But if it is, we
> probably have to just add some static inline functions in
> include/linux/kcov.h that simply does the check.

Yeah, this seems like a solution that will satisfy everyone. Will mail
a new version shortly.