Incorrect comparison of ticks in deadlkres

[Attilio Rao]

On Wed, May 29, 2013 at 1:18 AM, Ryan Stone <rysto32 at gmail.com> wrote:
> On Tue, May 28, 2013 at 5:29 PM, Ian Lepore <ian at freebsd.org> wrote:
>>
>> ticks is defined as a signed integer but conceptually it is unsigned --
>> it increments from 0 to UINT_MAX (not INT_MAX) then rolls over.  If
>> td->td_blktick is captured while ticks = UINT_MAX and later ticks has
>> rolled over and counted back up to 15, then ticks - td->td_blktick gives
>> an elapsed time of 16, as it should be.  Whether exploiting this
>> property of signed overflow is elegant or ugly is in the eye of the
>> beholder. :)
>>
>> If the intent of the "ticks < td->td_blktick" is to avoid the deadlock
>> check until "after enough time has passed," then I guess it should
>> probably be something more like "(ticks - td->blktick) > SOME_THRESHOLD"
>> so that it also uses the signed overflow trick.
>>
>> -- Ian
>
>
> It already does this later on to actually detect the deadlock.  The test is
> reversed but was intended to bail and not calculate the time elapsed at all
> if ticks had overflowed after td_blktick was captured, but as you say this
> is unnecessary.

I'm not sure if there was a comparison between the 2 values (ticks,
td_slpticks) somewhere, but if there is not and only add/sub to the
relative values then it is good to be removed.

Attilio


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