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

From: Marco Elver <elver@google.com>
To: Andrey Konovalov <andreyknvl@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: Fri, 13 Nov 2020 16:47:00 +0100	[thread overview]
Message-ID: <CANpmjNNd1-0+UX8Pkjov7Roq-c2RHcHed2znqvMVMxmoX-3LXg@mail.gmail.com> (raw)
In-Reply-To: <CAAeHK+yZEQ7r1bBWbUhdys8s1CntwpOyF+Fm+H=NiuK0g3KwYg@mail.gmail.com>

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.

Thanks,
-- Marco