[argobots-discuss] help understanding mutex_unlock logic

[Phil Carns]

I just opened a pull request that fixes the problem at 
https://github.com/pmodels/argobots/pull/22.  The issue ended up being a 
field that's not initialized properly when a thread is pushed onto a 
queue, but is needed by the pop logic later if there is more than one 
item in the queue.

I still feel like probably 
https://github.com/pmodels/argobots/blob/master/src/mutex.c#L802 should 
be an assertion rather than silently returning (a lock is held all the 
way from checking the element count to failing to find a thread, so it 
seems like an inconsistent scenario to hit that code path), but I did 
not address that in the pull request. I'd like someone to sanity check 
that part :)

thanks,
-Phil

On 08/14/2017 04:49 PM, Phil Carns wrote:
> I have a little bit more detail, but no root cause yet, on what's 
> going wrong here.  The ABTI_thread_queue data structure is definitely 
> corrupted in my case.  I've traced a call to ABTI_thread_htable_pop() 
> where (going into the function) p_queue->num_threads == 1, 
> p_queue->head == NULL, and p_queue->tail != NULL.
>
> There were 3 pushes and 2 pops before that point, so I think the 
> num_threads and tail variables are correct, but the head variable is 
> wrong.  The head and tail should both point to the same thing.
>
> Possibly a memory corruption in my own code, though.  I'll keep digging.
>
> thanks,
> -Phil
>
> On 08/11/2017 05:14 PM, Phil Carns wrote:
>> Hi all,
>>
>> I'm trying to debug a problem with a custom scheduler 
>> (https://xgitlab.cels.anl.gov/sds/abt-snoozer/issues/8).  To make a 
>> long story short, sometimes we can trigger a scenario where unlocking 
>> a mutex does not wake up any ULTs that are blocked on it.
>>
>> I've traced something suspicious in ABTI_mutex_wake_de() path leading 
>> up to the deadlock, but I'm not sure if this is an Argobots bug or if 
>> I just don't understand the logic.  Can someone help sanity check this?
>>
>> In the ABTI_mutex_wake_de() invocation just before the hang, the 
>> value of num_elem is 1 at this line:
>>
>> https://github.com/pmodels/argobots/blob/master/src/mutex.c#L769
>>
>> ... but then after checking the high and low priority lists it falls 
>> through to here, having not found anything to wake up:
>>
>> https://github.com/pmodels/argobots/blob/master/src/mutex.c#L802
>>
>> Is that code path supposed to be possible?
>>
>> It seems non intuitive that the count could  be > 0 but it can't find 
>> a thread to wake up, but there might be a more subtle meaning to 
>> num_elem.  The particular scheduler that I am debugging will be 
>> blocking/sleeping until it finds work to do, so it's important that 
>> this path ultimately triggers a pool push or else it won't make 
>> progress.  That may not be an issue with other schedulers.
>>
>> thanks!
>> -Phil
>>
>>
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