X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=Control%2FConcurrent%2FQSemN.hs;h=7e43908094d0888e82ab13cba20b8c308fdd6f13;hb=a0f41d471ceda7da65d775cfd9eaca3262ec8291;hp=991b0d562e4a59430f6423bdd16cf1d979dba6b0;hpb=45a54fd09e281c1b4eef3b610e90ea04e1def9b2;p=ghc-base.git

diff --git a/Control/Concurrent/QSemN.hs b/Control/Concurrent/QSemN.hs
index 991b0d5..7e43908 100644
--- a/Control/Concurrent/QSemN.hs
+++ b/Control/Concurrent/QSemN.hs
@@ -24,13 +24,14 @@ module Control.Concurrent.QSemN
 import Prelude
 
 import Control.Concurrent.MVar
+import Control.Exception ( mask_ )
 import Data.Typeable
 
 #include "Typeable.h"
 
 -- |A 'QSemN' is a quantity semaphore, in which the available
 -- \"quantity\" may be signalled or waited for in arbitrary amounts.
-newtype QSemN = QSemN (MVar (Int,[(Int,MVar ())]))
+newtype QSemN = QSemN (MVar (Int,[(Int,MVar ())])) deriving Eq
 
 INSTANCE_TYPEABLE0(QSemN,qSemNTc,"QSemN")
 
@@ -45,29 +46,30 @@ newQSemN initial =
 
 -- |Wait for the specified quantity to become available
 waitQSemN :: QSemN -> Int -> IO ()
-waitQSemN (QSemN sem) sz = do
+waitQSemN (QSemN sem) sz = mask_ $ do
   (avail,blocked) <- takeMVar sem   -- gain ex. access
-  if (avail - sz) >= 0 then
+  let remaining = avail - sz
+  if remaining >= 0 then
        -- discharging 'sz' still leaves the semaphore
        -- in an 'unblocked' state.
-     putMVar sem (avail-sz,blocked)
+     putMVar sem (remaining,blocked)
    else do
-     block <- newEmptyMVar
-     putMVar sem (avail, blocked++[(sz,block)])
-     takeMVar block
+     b <- newEmptyMVar
+     putMVar sem (avail, blocked++[(sz,b)])
+     takeMVar b
 
 -- |Signal that a given quantity is now available from the 'QSemN'.
 signalQSemN :: QSemN -> Int  -> IO ()
-signalQSemN (QSemN sem) n = do
+signalQSemN (QSemN sem) n = mask_ $ do
    (avail,blocked)   <- takeMVar sem
    (avail',blocked') <- free (avail+n) blocked
-   putMVar sem (avail',blocked')
+   avail' `seq` putMVar sem (avail',blocked')
  where
    free avail []    = return (avail,[])
-   free avail ((req,block):blocked)
+   free avail ((req,b):blocked)
      | avail >= req = do
-        putMVar block ()
+        putMVar b ()
         free (avail-req) blocked
      | otherwise    = do
         (avail',blocked') <- free avail blocked
-        return (avail',(req,block):blocked')
+        return (avail',(req,b):blocked')