Memory Barriers with STM and MVar

[Andrew Martin]

Thanks. The does have the information I was looking for. While
write_barrier gets used in a number of places, store_load_barrier never
actually gets used anywhere. This makes me suspect that MVar and STM cannot
be used to coordinate access to memory in this way my example tries to.
That is unfortunate.

On Fri, May 31, 2019 at 12:52 PM Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> what the precise guarantees are actually may depend on the platform.
>
> if you look for the barriers, they're in the RTS c codebase
>
> https://github.com/ghc/ghc/blob/bf73419518ca550e85188616f860961c7e2a336b/includes/stg/SMP.h#L24
> should get you started, theres also read_barrier and write_barrier in the
> rts
>
> On Tue, May 28, 2019 at 3:16 PM Andrew Martin <andrew.thaddeus at gmail.com>
> wrote:
>
>> Perhaps it may be better to open an issue for this information to be
>> added to the GHC user manual. Neither the word "barrier" nor the word
>> "fence" show up in the context of reordering in the user manual. Having
>> some kind of guarantee is extremely important for writing certain types of
>> applications. I suspect that both atomically and all the MVar operations
>> imply a full memory barrier, but I haven't been able this documented
>> anywhere.
>>
>> On Fri, May 24, 2019 at 1:38 PM Andrew Martin <andrew.thaddeus at gmail.com>
>> wrote:
>>
>>> I've been poking around in old threads and in the GHC source, and I
>>> cannot find this documented anywhere. What are the guarantees that GHC
>>> provides around load/store reordering when it comes to using MVar or
>>> TVar+retry+atomically. For example:
>>>
>>> import Data.Primitive
>>> import Control.Concurrent.MVar
>>> ...
>>> foo :: IO ()
>>> foo = do
>>>   ...
>>>   writeByteArray myArr 13 (42 :: Word)
>>>   putMVar myMVar ()
>>>
>>> If there is another thread that calls takeMVar followed by
>>> `readByteArray myArr 13`, is it guaranteed by GHC to see the 42 that's been
>>> written to the array. Same question applies for situations using STM
>>> facilities to accomplish blocking behavior that MVar gives us. Any
>>> documentation or pointers to places in GHC source where these barriers are
>>> guaranteed would be appreciated.
>>>
>>> --
>>> -Andrew Thaddeus Martin
>>>
>>
>>
>> --
>> -Andrew Thaddeus Martin
>> _______________________________________________
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>>
>

-- 
-Andrew Thaddeus Martin
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