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Date: Thu, 15 Dec 2016 08:44:46 +0100 (CET)
From: Julia Lawall <julia.lawall@lip6.fr>
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To: Nicholas Mc Guire <der.herr@hofr.at>
cc: Julia Lawall <julia.lawall@lip6.fr>,
        Nicholas Mc Guire <hofrat@osadl.org>,
        Gilles Muller <Gilles.Muller@lip6.fr>,
        Nicolas Palix <nicolas.palix@imag.fr>, Michal Marek <mmarek@suse.com>,
        Thomas Gleixner <tglx@linutronix.de>, Joe Perches <joe@perches.com>,
        cocci@systeme.lip6.fr, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2] Coccinelle: check usleep_range() usage
In-Reply-To: <20161215061624.GA23684@osadl.at>
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On Thu, 15 Dec 2016, Nicholas Mc Guire wrote:

> On Thu, Dec 15, 2016 at 06:52:28AM +0100, Julia Lawall wrote:
> >
> >
> > On Thu, 15 Dec 2016, Nicholas Mc Guire wrote:
> >
> > > Documentation/timers/timers-howto.txt outlines the intended usage of
> > > usleep_range(), this spatch tries to locate missuse/out-of-spec cases.
> > >
> > > Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
> > > ---
> > > V2: added context mode as suggested by Julia Lawall <julia.lawall@lip6.fr>
> > >     added min<max case sugested by Joe Perches <joe@perches.com>
> > >     added in the range checks as they are resonably reliable based on
> > >     a review of all 1648 call sites of usleep_range()
> > >
> > > 1648 calls total
> > > 1488 pass numeric values only (90.29%)
> > >   27 min below 10us (1.81%)
> > >   40 min above 10ms (2.68%)
> > >      min out of spec 4.50%
> > >   76 preprocessor constants (4.61%)
> > >    1 min below 10us (1.31%)
> > >    8 min above 10ms (10.52%)
> > >      min out of spec 11.84%
> > >   85 expressions (5.15%)
> > > 1(0) min below 10us (1.50%)*
> > > 6(2) min above 10ms (7.50%)*
> > >      min out of spec 9.0%
> > > Errors:
> > >   23 where min==max  (1.39%)
> > >    0 where max < min (0.00%)
> > >
> > > Total:
> > >   Bugs: 6.48%-10.70%*
> > >   Crit: 3.09%-3.15%* (min < 10, min==max, max < min)
> > >   Detectable by coccinelle:
> > >   Bugs: 74/103 (71.8%)
> > >   Crit: 50/52 (96.1%)
> > > * numbers estimated based on code review
> > >
> > > Patch is againts 4.9.0 (localversion-next is next-20161214)
> > >
> > >  scripts/coccinelle/api/bad_usleep_range.cocci | 88 +++++++++++++++++++++++++++
> > >  1 file changed, 88 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..003e9ef
> > > --- /dev/null
> > > +++ b/scripts/coccinelle/api/bad_usleep_range.cocci
> > > @@ -0,0 +1,88 @@
> > > +/// report bad/problematic usleep_range usage
> > > +//
> > > +// This is a checker for the documented intended use of usleep_range
> > > +// 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
> > > +//
> > > +// Limitations:
> > > +//  * The numeric limits are only checked when numeric constants are in
> > > +//    use (as of 4.9.0 thats 90.29% of the calls) no constant folding
> > > +//    is done - so this can miss some out-of-range cases - but in 4.9.0
> > > +//    it was catching 74 of the 103 bad cases (71.8%) and 50 of 52
> > > +//    (96.1%) of the critical cases (min < 10 and min==max - there
> > > +//  * There may be RT use-cases where both min < 10 and min==max)
> > > +//    justified (e.g. high-throughput drivers on a shielded core)
> > > +//
> > > +// 1) 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.
> > > +//
> > > +// 2) warn if min < 10 or min > 20ms
> > > +//
> > > +//  it makes little sense to use a non-atomic call for very short
> > > +//  delays because the scheduling jitter will most likely exceed
> > > +//  this limit - udelay() makes more sense in that case. For very
> > > +//  large delays using hrtimers is useless as preemption becomes
> > > +//  quite likely resulting in high inaccuracy anyway - so use
> > > +//  jiffies based msleep and don't burden the hrtimer subsystem.
> > > +//
> > > +// 3) warn if max < min
> > > +//
> > > +//  Joe Perches <joe@perches.com> added a check for this case
> > > +//  that is definitely wrong.
> > > +//
> > > +// Confidence: Moderate
> > > +// Copyright: (C) 2016 Nicholas Mc Guire, OSADL.  GPLv2.
> > > +// Comments:
> > > +// Options: --no-includes --include-headers
> > > +
> > > +virtual org
> > > +virtual report
> > > +virtual context
> > > +
> > > +@nullrangectx depends on context@
> > > +expression E1,E2;
> > > +position p;
> > > +@@
> > > +
> > > +* usleep_range@p(E1,E2)
> >
> > This is going to give a context warning on every call to usleep_range.
> > Why not E1,E1?
> yes this triggers on all use of usleep_ranges - as the report mode is
> checking for more than just min==max I thought its resonable to simply
> report all cases - maybe not.
> Not sure if it makes sense to add in the filter from below,

I think that simply flagging all the calls for context mode is not a good
idea.  Especilly since there is no explanation in this case.  If it is too
awkward to support context mode, just drop it.

julia

>
> Thre actually are quite a few bad use patters beyond these basic ones
> like
>                 unsigned long timeout = jiffies + msecs_to_jiffies(250);
>
> -               while (time_before(jiffies, timeout)) {
>                         value = fpci_readl(tegra, XUSB_CFG_ARU_MBOX_OWNER);
>                         if (value == MBOX_OWNER_NONE)
>                                 break;
>
> -                       usleep_range(10, 20);
>                 }
>
> >
> > > +
> > > +
> > > +@nullrange@
> > > +expression E1,E2;
> > > +position p;
> > > +@@
> > > +
> > > +  usleep_range@p(E1,E2)
> > > +
> > > +@script:python depends on !context@
> > > +p << nullrange.p;
> > > +min << nullrange.E1;
> > > +max << nullrange.E2;
> > > +@@
> > > +
> > > +if(min == max):
> > > +   msg = "WARNING: usleep_range min == max (%s) - consider delta " % (min)
> > > +   coccilib.report.print_report(p[0], msg)
> > > +if str.isdigit(min):
> >
> > I guess this checks if min is a constant, but doesn't the last case also
> > need to check if max is a constant?
> >
> yes it does - seems that there simply was no
> such case so it went unnoticed.
>
> also just noticed that the org mode would also be using
> coccilib.report rather than coccilib.org...
>
> thx!
> hofrat
>
>