Don't move Capabilities in setNumCapabilities (#8209)

We have various problems with reallocating the array of Capabilities,
due to threads in waitForReturnCapability that are already holding a
pointer to a Capability.

Rather than add more locking to make this safer, I decided it would be
easier to ensure that we never move the Capabilities at all.  The
capabilities array is now an array of pointers to Capabaility.  There
are extra indirections, but it rarely matters - we don't often access
Capabilities via the array, normally we already have a pointer to
one.  I ran the parallel benchmarks and didn't see any difference.

1 files changed, 5 insertions, 5 deletions

diff --git a/rts/sm/Storage.c b/rts/sm/Storage.c
index a5337bc5b2..b575fc3e52 100644
--- a/rts/sm/Storage.c
+++ b/rts/sm/Storage.c
@@ -215,7 +215,7 @@ void storageAddCapabilities (nat from, nat to)
     // we've moved the nurseries, so we have to update the rNursery
     // pointers from the Capabilities.
     for (i = 0; i < to; i++) {
-        capabilities[i].r.rNursery = &nurseries[i];
+        capabilities[i]->r.rNursery = &nurseries[i];
     }
 
     /* The allocation area.  Policy: keep the allocation area
@@ -229,7 +229,7 @@ void storageAddCapabilities (nat from, nat to)
     // allocate a block for each mut list
     for (n = from; n < to; n++) {
         for (g = 1; g < RtsFlags.GcFlags.generations; g++) {
-            capabilities[n].mut_lists[g] = allocBlock();
+            capabilities[n]->mut_lists[g] = allocBlock();
         }
     }
 
@@ -493,8 +493,8 @@ assignNurseriesToCapabilities (nat from, nat to)
     nat i;
 
     for (i = from; i < to; i++) {
-        capabilities[i].r.rCurrentNursery = nurseries[i].blocks;
-        capabilities[i].r.rCurrentAlloc   = NULL;
+        capabilities[i]->r.rCurrentNursery = nurseries[i].blocks;
+        capabilities[i]->r.rCurrentAlloc   = NULL;
     }
 }
 
@@ -939,7 +939,7 @@ void updateNurseriesStats (void)
     nat i;
 
     for (i = 0; i < n_capabilities; i++) {
-        capabilities[i].total_allocated += countOccupied(nurseries[i].blocks);
+        capabilities[i]->total_allocated += countOccupied(nurseries[i].blocks);
     }
 }