Re: [PATCH] Coccinelle: uslee_range: ensure delta not zero

[Nicholas Mc Guire]

On Tue, Dec 13, 2016 at 01:09:38PM +0100, Julia Lawall wrote:
> 
> 
> On Tue, 13 Dec 2016, Nicholas Mc Guire wrote:
> 
> > usleep_range() min==max makes little sense at last for non-RT, so issue
> > a warning if delta is 0.
> >
> > Signed-off-by: Nicholas Mc Guire <hofrat@xxxxxxxxx>
> > ---
> >
> > As of 4.9.0 this finds about 20 cases - all of which look like the
> > should be passing a range.
> >
> > Patch is against 4.9.0 (localversion-next is next-20161213)
> >
> >  scripts/coccinelle/api/bad_usleep_range.cocci | 55 +++++++++++++++++++++++++++
> >  1 file changed, 55 insertions(+)
> >  create mode 100644 scripts/coccinelle/api/bad_usleep_range.cocci
> >
> > diff --git a/scripts/coccinelle/api/bad_usleep_range.cocci b/scripts/coccinelle/api/bad_usleep_range.cocci
> > new file mode 100644
> > index 0000000..7e05f3e
> > --- /dev/null
> > +++ b/scripts/coccinelle/api/bad_usleep_range.cocci
> > @@ -0,0 +1,55 @@
> > +/// bad uslee_range - warn if min == max
> > +//
> > +//The problem is that usleep_range is calculating the delay by
> > +//      exp = ktime_add_us(ktime_get(), min)
> > +//      delta = (u64)(max - min) * NSEC_PER_USEC
> > +//so delta is set to 0 if min==max
> > +//and then calls
> > +//      schedule_hrtimeout_range(exp, 0,...)
> > +//effectively this means that the clock subsystem has no room to
> > +//optimize. usleep_range() is in non-atomic context so a 0 range
> > +//makes very little sense as the task can be preempted anyway so
> > +//there is no guarantee that the 0 range would be adding much
> > +//precision - it just removes optimization potential, so it probably
> > +//never really makes sense for any non-RT systems.
> > +//
> > +//see: Documentation/timers/timers-howto.txt and
> > +//Link: http://lkml.org/lkml/2016/11/29/54 for some notes on
> > +//      when mdelay might not be a suitable replacement
> > +//
> > +// Confidence: Moderate
> > +// Copyright: (C) 2016 Nicholas Mc Guire, OSADL.  GPLv2.
> > +// Comments:
> > +// Options: --no-includes --include-headers
> > +
> > +virtual org
> > +virtual report
> > +
> > +@nullrange@
> > +expression E;
> > +constant C;
> > +position p;
> > +@@
> > +
> > +<+...
> > +(
> > +  usleep_range@p(C,C)
> > +|
> > +  usleep_range@p(E,E)
> > +)
> > +...+>
> 
> The outer <+... ...+> is not needed.
> 
> You could support context too.
> 
> The E,E case subsumes the C,C case.  Unless you want to put different
> messages for the two cases, there is no need for both of them.
>
ok will kick that - in my script I was also doing range checks
on constants to distinuish between the udelay range < 10us and
the msleep* range > 10ms - but that is not as clear a case as the
min==max case - will remove the constant case - the unnecessary
<+... ...+> just means I still did not understand when it is actually needed
will remove that as well.

thx!
hofrat