MVar primops are not cheking for stack correctly.

Summary

We use stack space without checking for stack size first.

readMVar will call stg_readMVarzh implemented in PrimOps.cmm. This function itself doesn't use stack.

stg_readMVarzh will call stg_block_readmvar in HeapStackCheck.cmm:

stg_block_readmvar /* mvar passed in R1 */
{
    Sp_adj(-2);
    Sp(1) = R1;
    Sp(0) = stg_block_readmvar_info;
    R3 = R1; // mvar communicated to stg_block_readmvar_finally in R3
    BLOCK_BUT_FIRST(stg_block_readmvar_finally);
}

//Compiles to:

stg_block_readmvar() { //  []
         { info_tbls: []
           stack_info: arg_space: 8 updfr_space: Just 8
         }
     {offset
       c2F:
           Sp = Sp - 16;
           I64[Sp + 8] = R1;
           I64[Sp] = stg_block_readmvar_info;
           R3 = R1;
           I16[CurrentTSO + 32] = 1 :: W16;
           I64[BaseReg + 912] = 4;
           R1 = BaseReg;
           R2 = stg_block_readmvar_finally;
           call stg_returnToSchedButFirst(R1, R2, R3) args: 8, res: 0, upd: 8;
     }

Which clearly doesn't perform a stack check when it should.

This was identified by @TerrorJack on the ghc-devs mailing list. I merely confirmed his suspicions and reported it here.

The correct fix is to insert a stack check manually.