Fix #3429: a tricky race condition

There were two bugs, and had it not been for the first one we would
not have noticed the second one, so this is quite fortunate.

The first bug is in stg_unblockAsyncExceptionszh_ret, when we found a
pending exception to raise, but don't end up raising it, there was a
missing adjustment to the stack pointer.  

The second bug was that this case was actually happening at all: it
ought to be incredibly rare, because the pending exception thread
would have to be killed between us finding it and attempting to raise
the exception.  This made me suspicious.  It turned out that there was
a race condition on the tso->flags field; multiple threads were
updating this bitmask field non-atomically (one of the bits is the
dirty-bit for the generational GC).  The fix is to move the dirty bit
into its own field of the TSO, making the TSO one word larger (sadly).

1 files changed, 10 insertions, 0 deletions

diff --git a/rts/Exception.cmm b/rts/Exception.cmm
index 16b5d92b30..cd3f2bc4b8 100644
--- a/rts/Exception.cmm
+++ b/rts/Exception.cmm
@@ -49,6 +49,8 @@ import ghczmprim_GHCziBool_True_closure;
 
    -------------------------------------------------------------------------- */
 
+STRING(stg_unblockAsync_err_str, "unblockAsyncExceptions#_ret")
+
 INFO_TABLE_RET( stg_unblockAsyncExceptionszh_ret, RET_SMALL )
 {
     CInt r;
@@ -81,6 +83,14 @@ INFO_TABLE_RET( stg_unblockAsyncExceptionszh_ret, RET_SMALL )
                 jump %ENTRY_CODE(Sp(0));
             }
         }
+        else {
+            /*
+               the thread might have been removed from the
+               blocked_exception list by someone else in the meantime.
+               Just restore the stack pointer and continue.  
+            */   
+            Sp_adj(2);
+        }
     }
 
     Sp_adj(1);