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Date: Sat, 25 May 2013 17:55:09 -0400
Message-ID: <CAFMmRNyQCs-yOB7gm4TRq3xcMp50PEJc0YNQLAjMs3q8iE-ZUw@mail.gmail.com>
Subject: Incorrect comparison of ticks in deadlkres
From: Ryan Stone <rysto32@gmail.com>
To: FreeBSD Current <freebsd-current@freebsd.org>
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Currently deadlkres performs the following comparison when trying to check
for threads that have been blocked on a mutex or sleeping on an sx lock:

if (TD_ON_LOCK(td) && ticks < td->td_blktick) {
    /* check for deadlock...*/


The test against ticks is incorrect.  It results in deadlkres only
signaling a deadlock after ticks has rolled over; at 1000 hz this will take
up to 49 days.  From looking at the history of the code this test appears
to be a attempt to deal with ticks rollover.  However this is necessary;
later on the code calculates the amount of time that has passed with:

tticks = ticks - td->td_blktick;

ticks was designed to exploit integer underflow in the case of rollover to
guarantee that subtraction produces correct results in all cases (other
than a double rollover, of course).  I am going to remove the two incorrect
tests unless somebody can point out a overflow/underflow case that I
haven't considered.