[commit: ghc] ghc-7.10: Fix for crash in setnumcapabilities001 (6de9b6e)

[git at git.haskell.org]

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

On branch  : ghc-7.10
Link       : http://ghc.haskell.org/trac/ghc/changeset/6de9b6eb59c7525255e6780c57fa361ec108fc88/ghc

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

commit 6de9b6eb59c7525255e6780c57fa361ec108fc88
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Fri Jun 19 14:41:32 2015 +0100

    Fix for crash in setnumcapabilities001
    
    getNewNursery() was unconditionally incrementing next_nursery, which
    is normally fine but it broke an assumption in
    storageAddCapabilities().  This manifested as an occasional crash in
    the setnumcapabilities001 test.
    
    (cherry picked from commit be0ce8718ea40b091e69dd48fe6bc62b6b551154)


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

6de9b6eb59c7525255e6780c57fa361ec108fc88
 rts/sm/Storage.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/rts/sm/Storage.c b/rts/sm/Storage.c
index 50926b7..297e584 100644
--- a/rts/sm/Storage.c
+++ b/rts/sm/Storage.c
@@ -549,6 +549,7 @@ allocNursery (bdescr *tail, W_ blocks)
 STATIC_INLINE void
 assignNurseryToCapability (Capability *cap, nat n)
 {
+    ASSERT(n < n_nurseries);
     cap->r.rNursery = &nurseries[n];
     cap->r.rCurrentNursery = nurseries[n].blocks;
     newNurseryBlock(nurseries[n].blocks);
@@ -699,14 +700,19 @@ resizeNurseries (W_ blocks)
 rtsBool
 getNewNursery (Capability *cap)
 {
-    StgWord i = atomic_inc(&next_nursery, 1) - 1;
-    if (i >= n_nurseries) {
-        return rtsFalse;
+    StgWord i;
+
+    for(;;) {
+        i = next_nursery;
+        if (i >= n_nurseries) {
+            return rtsFalse;
+        }
+        if (cas(&next_nursery, i, i+1) == i) {
+            assignNurseryToCapability(cap, i);
+            return rtsTrue;
+        }
     }
-    assignNurseryToCapability(cap, i);
-    return rtsTrue;
 }
-
 /* -----------------------------------------------------------------------------
    move_STACK is called to update the TSO structure after it has been
    moved from one place to another.