No atomic read on MVar?

3 Nov 2008

      Dear GHCers,

I ran face first into an assumption I had made on MVar operations (in 
Control.Concurrent); I had assumed there to be an atomic read (i.e. 
non-destructive read, as opposed to destructive consume/take). The following 
program illustrates what I had in mind.

testAtomic :: IO ()
testAtomic = do
	var <- newMVar 0
	putStrLn("Fork")
	forkIO (putMVar var 1 >> putStrLn "X")
	yield
	r1 <- readMVar var
	putStrLn("1")
	r2 <- takeMVar var
	putStrLn("2")
	r3 <- takeMVar var
	putStrLn("Result: " ++ show [r1,r2,r3])

If readMVar had been atomic, the result would be program termination with a 
result of [0,0,1] being output. However, readMVar simply combines takeMVar 
and putMVar, so the reading of r1 blocks after the takeMVar, because upon 
taking the MVar, the blocked thread wakes up, puts 1 in var and prints X. 
readMVar does not terminate for r1 (i.e. "1" is never printed).

I have now implemented my variable as a pair of MVars, one of which serves as 
a lock on the other. Both for performance reasons and for deadlock analysis, 
I would really like an atomic read on MVars, though. Does it exist? If not, 
why not?

Regards,
Philip

Philip K.F. Hölzenspies

Simon Marlow

David Menendez

Arnar Birgisson

Chris Kuklewicz

Philip K.F. Hölzenspies

tags

participants (5)