On 10/10/2011 09:04, Simon Peyton-Jones wrote:
Thank you for the detailed investigation. I have not followed all the
details of this thread, but I *think* that it may (happily) represent a
bug in generating TypeReps that is already fixed.
·We used to have a global cache from which we generated unique Int keys
corresponding to type constructors. The trouble with this was that (a)
you weren’t guaranteed to get the same key in every run, and (b) the
cache was not initially designed to be thread-safe, and I’m not sure
that we’d closed all race conditions.
Indeed, I don't think we closed *any* race conditions. The code looks
completely unthreadsafe to me. There's a non-atomic lookup and update
in the cache, and a non-atomic genSym to get new keys. It's pretty bad
that we didn't notice this or fix it earlier.
Sorry about the bug. As Simon said, 7.2 and later should fix it.
Cheers,
Simon
·But NOW we generate a MD5 hash, or fingerprint, of the type. So there
is no global cache, no race condition, and you should get the same
behaviour ever time.
In short, can you try with 7.2?
Thanks
Simon
*From:*glasgow-haskell-users-boun...@haskell.org
[mailto:glasgow-haskell-users-boun...@haskell.org] *On Behalf Of
*Jean-Marie Gaillourdet
*Sent:* 09 October 2011 12:53
*To:* glasgow-haskell-users
*Subject:* Is this a concurrency bug in base?
Hi,
I am working on a library I'd like to release to hackage very soon, but
I've found a problem with supporting GHC 6.12 and GHC 7.0.
Consider the following program:
import Control.Concurrent
import Data.Typeable
main :: IO ()
main =
do { fin1 <- newEmptyMVar
; fin2 <- newEmptyMVar
; forkIO $ typeRepKey (typeOf ()) >>= print >> putMVar fin1 ()
; forkIO $ typeRepKey (typeOf ()) >>= print >> putMVar fin2 ()
; () <- takeMVar fin1
; () <- takeMVar fin2
; putStrLn "---"
; return ()
}
When compiled with GHC 7.0.x or GHC 6.12.x, it should print two
identical numbers. Sometimes it does not.
To reproduce this compile and execute as follows:
$ ghc-7.0.3 -rtsopts -threaded TypeRepKey.hs -o TypeRepKey
$ while true ; do ./TypeRepKey +RTS -N ; done
0
0
---
0
0
---
0
0
---
0
1
---
0
0
---
…
Ideally you should get an infinite number of zeros but once in a while
you have a single(!) one in between. The two numbers of one program run
should be identical, but their values may be arbitrary. But it should
not be possible to have single outliers.
This only happens when executed with more than one thread. I've also a
somewhat larger program which seems to indicate that fromDynamic fails
occasionally. I can post it as well if it helps. This seems to be a
Heisenbug as it is extremely fragile, when adding a "| grep 1" to the
while loop it seems to disappears. At least on my computers.
All this was done on several Macs running the latest OS X Lion with ghc
7.0.3 from the binary distribution on the GHC download page.
Actually, I am trying to find a method to use a "type" as key in a map
which works before GHC 7.2. I'd be glad to get any ideas on how to
achieve that, given that typeRepKey seems to buggy.
I'd be happy to get any comments on this matter.
Regards,
Jean
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