Message-ID: <20150429232721.59b89cb1@pog.tecnopolis.ca>
Date:	Wed, 29 Apr 2015 23:27:21 -0500
From:	Trevor Cordes <trevor@...nopolis.ca>
To:	linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Josh Boyer <jwboyer@...hat.com>
Subject: Re: regression in ktime.h circa 3.16.0-rc5+ breaks lirc irsend, bad
 commit 166afb64511

Sorry for the top-posting; Josh Boyer suggested I re-mail this mail
from last month which didn't get any replies.  I'm still having this
weird kernel bug affecting me and I've bisected it down to like 2-4
lines of code.  (I've thought more about my theory regarding
unsigned/signed below and it's probably wrong, so ignore my
prognosticating.)  Please see my rhbz link near the bottom for the full
details.

Thanks everyone!

On 2015-03-23 Trevor Cordes wrote:
> Hello everyone, this is my first attempt at bisecting a kernel to
> solve a bug.  Please bear with me.
> 
> I have successfully bisected and located a commit that is causing my 
> problem.  Look at commit 166afb64511.
> 
> ktime_to_us returns s64, but the commit changes it so ktime_to_us
> just returns what ktime_divns returns, and ktime_divns returns a
> u64!  If the u64 is big enough, wouldn't it wrap s64 around to a
> negative number?  Or, perhaps if some caller is passing in negative
> ktime_t to begin with it will trigger without having to hit big
> numbers.  With my limited knowledge of C, I am stabbing in the dark
> here.
> 
> That's just my guess as to why this commit causes my problem.  My bug 
> symptom is my previously working MythTV lirc blaster no longer
> reliably sends IR signals.  Using irsend to test I can see irsend is
> just timing out (and only sometimes blasts, usually the first
> attempt).  On good kernels it returns immediately after blasting.
> 
> This little patch (at bottom of email) that puts the code back in
> place and gets rid of the function call fixes the problem for me.  I
> applied this patch to the very latest FC21
> kernel-PAE-3.19.1-201.fc21.i686 src.rpm and rpmbuilded and the bug is
> gone!  I can once again MythTV.  Hooray.
> 
> I suspect no one else is seeing this because less people are running 
> 32-bit now, and perhaps in most code paths the value of the u64 never
> gets above 2^63.  I suspect something in drivers/media (possibly) is
> passing very high or negative values (possibly another bug) to these
> calls.
> 
> Obviously my patch isn't the real solution, the real solution is to
> make the new function calls use a consistent 64-bit type, or figure
> out what in my code path is calling these functions and check it for
> value sanity.
> 
> I've documented the whole process / details of this bug in RHBZ:
> https://bugzilla.redhat.com/show_bug.cgi?id=1200353
> 
> Thanks!
> 
> diff -uNr a/include/linux/ktime.h b/include/linux/ktime.h
> --- a/include/linux/ktime.h	2015-02-08 20:54:22.000000000 -0600
> +++ b/include/linux/ktime.h	2015-03-23 01:09:43.000000000 -0500
> @@ -173,12 +173,16 @@
>  
>  static inline s64 ktime_to_us(const ktime_t kt)
>  {
> -	return ktime_divns(kt, NSEC_PER_USEC);
> +/*	return ktime_divns(kt, NSEC_PER_USEC); */
> +	struct timeval tv = ktime_to_timeval(kt);
> +	return (s64) tv.tv_sec * USEC_PER_SEC + tv.tv_usec;
>  }
>  
>  static inline s64 ktime_to_ms(const ktime_t kt)
>  {
> -	return ktime_divns(kt, NSEC_PER_MSEC);
> +/*	return ktime_divns(kt, NSEC_PER_MSEC); */
> +	struct timeval tv = ktime_to_timeval(kt);
> +	return (s64) tv.tv_sec * MSEC_PER_SEC + tv.tv_usec /
> USEC_PER_MSEC; }
>  
>  static inline s64 ktime_us_delta(const ktime_t later, const ktime_t
> earlier)

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