Re: [PATCH v4] kcov, usb: only collect coverage from __usb_hcd_giveback_urb in softirq

From: Andrey Konovalov <andreyknvl@google.com>
To: Marco Elver <elver@google.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Dmitry Vyukov <dvyukov@google.com>,
	USB list <linux-usb@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	Shuah Khan <shuah@kernel.org>,
	Alexander Potapenko <glider@google.com>,
	Aleksandr Nogikh <nogikh@google.com>,
	Nazime Hande Harputluoglu <handeharput@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v4] kcov, usb: only collect coverage from __usb_hcd_giveback_urb in softirq
Date: Tue, 24 Nov 2020 00:42:27 +0100	[thread overview]
Message-ID: <CAAeHK+wC4uHz0-g8jEp=Nn11De1EpDsNeOLbXxrR2hJ3-2DFkA@mail.gmail.com> (raw)
In-Reply-To: <CANpmjNNd1-0+UX8Pkjov7Roq-c2RHcHed2znqvMVMxmoX-3LXg@mail.gmail.com>

On Fri, Nov 13, 2020 at 4:47 PM Marco Elver <elver@google.com> wrote:
>
> On Fri, 13 Nov 2020 at 14:42, Andrey Konovalov <andreyknvl@google.com> wrote:
> > On Fri, Nov 13, 2020 at 2:28 PM Sebastian Andrzej Siewior
> > <bigeasy@linutronix.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.