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<p style="margin: 10px 0 0 0">I'm going to need help from Scot with this one.</p>
<p style="margin: 10px 0 0 0">I spent some time this morning looking at this and I just cannot see the inversion. I'm perfectly happy to believe that it exists but there's just too much I don't grock (yet).</p>
<p style="margin: 10px 0 0 0">Firstly the initial post was about shibd shutdown hanging as it waits for the relaod thread to exit (which seems feasible if there is something odd with the locking of the reloader) and the example is inside apache - and doesn't appear to have anything to do with the reload thread.</p>
<p style="margin: 10px 0 0 0">I don't speak gdb but it seems to me that the fact provided example seems to show two thread queueing for a lock but it doesn't show inversion. In order to see inversion I need to see 2 locks and 2 threads with one held by each and queueing for the other. A single lock inversion might be a shared (read) lock being upconverted to a exclusive (write) lock without the shared bit being dropped first <span class="error">[and don't ask me how much of my life was spend debugging issues in the VMS filesystem where there was a call to do that only it didn't always work right]</span> but I don't see that either.</p>
<p style="margin: 10px 0 0 0">Most of all I don't buy (yet) that ripping the locking dance out of the ->lock() call will fix anything. It can certainly reduce the chances of deadlock but not remove it.</p>
<p style="margin: 10px 0 0 0">This is because for a normal 2 (or greater) lock inversion deadlock, the mode of the lock (read or write) is irrelevant - absent an implementation which allows for AquireReadStarveWrite, which AIUI pthreads doesn't necessarily have.</p>
<p style="margin: 10px 0 0 0">If you can get a deadlock between a thread with a lock held for Write and another thread waiting for the lock for Read then you can get exactly the same deadlock between a thread with the lock held for read and another thread waiting for the lock for read - you do this by introducing a random thread which is queuing for the lock for Write. Then the thread waiting for read will block, just as it would if the other thread had the lock for Write so the same deadlock. So the fact that the lock is held for write when lock exits cannot matter - what matters is the inversion.</p>
<p style="margin: 10px 0 0 0">So I'm stumped and looking for elucidation.</p>
<p style="margin: 10px 0 0 0">But I do have an apache-based shib install in a (Windows) VM so I should be able to repro this deadlock myself; always assuming that the Windows pthread implementation has the same semantics as RHEL (which seems unlikely).</p>
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