[commit: ghc] master: Don't lock the MVar closure on tryReadMVar (6bb0c5d)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6bb0c5db818c1ba9cd5fe1785a3020cfddf0c223/ghc

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

commit 6bb0c5db818c1ba9cd5fe1785a3020cfddf0c223
Author: David Feuer <david.feuer at gmail.com>
Date:   Fri Jun 29 14:31:33 2018 -0400

    Don't lock the MVar closure on tryReadMVar
    
    It shouldn't be necessary to lock the `MVar` closure on `tryReadMVar`, since it
    just reads one field of the structure and doesn't make any modifications. So
    let's not.
    
    Reviewers: bgamari, erikd, simonmar, fryguybob, osa1
    
    Reviewed By: osa1
    
    Subscribers: osa1, rwbarton, thomie, carter
    
    Differential Revision: https://phabricator.haskell.org/D4905


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

6bb0c5db818c1ba9cd5fe1785a3020cfddf0c223
 rts/PrimOps.cmm | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/rts/PrimOps.cmm b/rts/PrimOps.cmm
index 6081fab..058fe1e 100644
--- a/rts/PrimOps.cmm
+++ b/rts/PrimOps.cmm
@@ -1825,18 +1825,14 @@ stg_readMVarzh ( P_ mvar, /* :: MVar a */ )
 
 stg_tryReadMVarzh ( P_ mvar, /* :: MVar a */ )
 {
-    W_ val, info, tso, q;
+    W_ val;
 
-    LOCK_CLOSURE(mvar, info);
+    val = StgMVar_value(mvar);
 
-    if (StgMVar_value(mvar) == stg_END_TSO_QUEUE_closure) {
-        unlockClosure(mvar, info);
+    if (val == stg_END_TSO_QUEUE_closure) {
         return (0, stg_NO_FINALIZER_closure);
     }
 
-    val = StgMVar_value(mvar);
-
-    unlockClosure(mvar, info);
     return (1, val);
 }