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Date: Thu, 15 Dec 2016 06:52:28 +0100 (CET)
From: Julia Lawall <julia.lawall@lip6.fr>
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To: Nicholas Mc Guire <hofrat@osadl.org>
cc: 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
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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?

> +
> +
> +@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?

julia


> +   if(int(min) < 10):
> +      msg = "ERROR: usleep_range min (%s) less than 10us - consider using udelay()" % (min)
> +      coccilib.report.print_report(p[0], msg)
> +   if(20000 < int(min)):
> +      msg = "ERROR: usleep_range min (%s) exceed 20m - consider using mslee()" % (min)
> +      coccilib.report.print_report(p[0], msg)
> +   if(int(max) < int(min)):
> +      msg = "ERROR: usleep_range max (%s) less than min (%s)" % (max,min)
> +      coccilib.report.print_report(p[0], msg)
> --
> 2.1.4
>
>