[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

[Brandon Allbery]

Actually, the current -n looks like a generalization of what I remember
being there --- and one that re-adds safety, since instead of "scavenging"
off another thread's nursery, the exhausting thread gets another chunk of
nursery for itself if one is available.

On Wed, Nov 22, 2017 at 2:25 PM, Brandon Allbery <allbery.b at gmail.com>
wrote:

> There's some other complications here as well. Threads by default get
> their own nurseries, so you have such a safe window even in threaded
> programs --- but if another thread allocates enough (and subject to RTS
> options, IIRC -n matters here?) it could fill its nursery, and the RTS
> notices your thread's nursery is mostly unused and let the other thread
> start allocating from your nursery and shrink your safe window.
>
> (Also, it looks to me like 8.2's runtime changed some things; I thought
> there was an option explicitly specifying/controlling the
> separate-nurseries behavior, but I didn't see it in checking through the
> runtime options just now.)
>
> On Wed, Nov 22, 2017 at 2:15 PM, Brandon Allbery <allbery.b at gmail.com>
> wrote:
>
>> Actually, I'd say that it can, but only if used incorrectly. Which is why
>> it's reallyUnsafe.
>>
>> The OP is correct in that a gc at the wrong time can lead to spurious
>> positives. The key to this is that, if you force everything to be evaluated
>> to WHNF (so you actually have the pointers) and then gc, you have some
>> determinacy as to when the next gc will happen: force to WHNF first, gc,
>> make sure any subsequent operations between that and your
>> reallyUnsafePtrEquality either don't allocate or fit in the nursery --- and
>> in this case, you can trust the result. So it requires a fair amount of
>> care, but there is a window in which it is safe.
>>
>> On Wed, Nov 22, 2017 at 2:06 PM, Andrew Martin <andrew.thaddeus at gmail.com
>> > wrote:
>>
>>> It cannot give false positives. If it could, that would make it totally
>>> worthless.
>>>
>>> Sent from my iPhone
>>>
>>> > On Nov 22, 2017, at 12:22 PM, Michael Walker <mike at barrucadu.co.uk>
>>> wrote:
>>> >
>>> > Hello,
>>> >
>>> > Can reallyUnsafePtrEquality give false positives?  I can see how it
>>> > can give false negatives (eg, compiler optimisations increasing or
>>> > decreasing sharing), but I'm not so sure if it can give false
>>> > positives.
>>> >
>>> > I don't see how in a garbage collected language two live values could
>>> > compare reference equal.  Unless the implementation is something like:
>>> >
>>> > reallyUnsafePtrEquality a b = getptr a == getptr b
>>> >
>>> > ...as that then means that if the GC moves things after `getptr a` is
>>> > evaluated but before `getptr b` is, then you could get a false
>>> > positive.  But that doesn't seem like a sensible implementation to me,
>>> > because then reallyUnsafePtrEquality would surely be totally useless.
>>> >
>>> > --
>>> > Michael Walker (http://www.barrucadu.co.uk)
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>>
>>
>>
>> --
>> brandon s allbery kf8nh                               sine nomine
>> associates
>> allbery.b at gmail.com
>> ballbery at sinenomine.net
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>>
>
>
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>



-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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