[commit: ghc] wip/tdammers/T11066: Unmask readMVar in readChan (35e5106)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : wip/tdammers/T11066
Link       : http://ghc.haskell.org/trac/ghc/changeset/35e510699dd02bd157a1ceabb0c9495f70665eb7/ghc

>---------------------------------------------------------------

commit 35e510699dd02bd157a1ceabb0c9495f70665eb7
Author: David Feuer <david.feuer at gmail.com>
Date:   Tue May 29 16:51:16 2018 -0400

    Unmask readMVar in readChan
    
    When `readMVar` was implemented using `takeMVar` and `putMVar`,
    we needed to use `modifyMVarMasked` in `readChan` just in case
    the `readMVar` was interrupted between taking and putting. Now
    that `readMVar` uses an atomic primop, this is impossible, so we can
    safely unmask `readMVar`.
    
    Reviewers: hvr, bgamari, simonmar
    
    Reviewed By: simonmar
    
    Subscribers: rwbarton, thomie, carter
    
    Differential Revision: https://phabricator.haskell.org/D4738


>---------------------------------------------------------------

35e510699dd02bd157a1ceabb0c9495f70665eb7
 libraries/base/Control/Concurrent/Chan.hs | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/libraries/base/Control/Concurrent/Chan.hs b/libraries/base/Control/Concurrent/Chan.hs
index d752a89..d0fed4a 100644
--- a/libraries/base/Control/Concurrent/Chan.hs
+++ b/libraries/base/Control/Concurrent/Chan.hs
@@ -106,21 +106,12 @@ writeChan (Chan _ writeVar) val = do
 -- thread holds a reference to the channel.
 readChan :: Chan a -> IO a
 readChan (Chan readVar _) = do
-  modifyMVarMasked readVar $ \read_end -> do -- Note [modifyMVarMasked]
+  modifyMVar readVar $ \read_end -> do
     (ChItem val new_read_end) <- readMVar read_end
         -- Use readMVar here, not takeMVar,
         -- else dupChan doesn't work
     return (new_read_end, val)
 
--- Note [modifyMVarMasked]
--- This prevents a theoretical deadlock if an asynchronous exception
--- happens during the readMVar while the MVar is empty.  In that case
--- the read_end MVar will be left empty, and subsequent readers will
--- deadlock.  Using modifyMVarMasked prevents this.  The deadlock can
--- be reproduced, but only by expanding readMVar and inserting an
--- artificial yield between its takeMVar and putMVar operations.
-
-
 -- |Duplicate a 'Chan': the duplicate channel begins empty, but data written to
 -- either channel from then on will be available from both.  Hence this creates
 -- a kind of broadcast channel, where data written by anyone is seen by