diff options
| -rw-r--r-- | rts/PrimOps.cmm | 24 | ||||
| -rw-r--r-- | rts/sm/NonMoving.c | 4 |
2 files changed, 28 insertions, 0 deletions
diff --git a/rts/PrimOps.cmm b/rts/PrimOps.cmm index e073b66ae8..41b3d269bf 100644 --- a/rts/PrimOps.cmm +++ b/rts/PrimOps.cmm @@ -1545,6 +1545,23 @@ stg_writeTVarzh (P_ tvar, /* :: TVar a */ * exception and never perform its take or put, and we'd end up with a * deadlock. * + * Note [Nonmoving write barrier in Perform{Take,Put}] + * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + * As noted in Note [Non-moving garbage collector] in NonMoving.c, the + * non-moving GC requires that all overwritten pointers be pushed to the update + * remembered set. In the case of stack mutation this typically happens by + * "dirtying" the stack, which eagerly traces the entire stack chunk. + * + * An exception to this rule is PerformPut, which mutates the stack of a + * blocked thread (overwriting an stg_block_putmvar frame). To ensure that the + * collector sees the MVar and value reachable from the overwritten frame, we + * must push them to the update remembered set. Failing to do so was the cause + * of #20399. + * + * Note that unlike PerformPut, the callers of PerformTake first dirty the + * stack prior mutating it (since they introduce a *new*, potentially + * inter-generational reference to the stack) and therefore the barrier + * described above is unnecessary in this case. * -------------------------------------------------------------------------- */ stg_isEmptyMVarzh ( P_ mvar /* :: MVar a */ ) @@ -1573,15 +1590,22 @@ stg_newMVarzh () } +// See Note [Nonmoving write barrier in Perform{Put,Take}]. +// Precondition: the stack must be dirtied. #define PerformTake(stack, value) \ W_ sp; \ sp = StgStack_sp(stack); \ W_[sp + WDS(1)] = value; \ W_[sp + WDS(0)] = stg_ret_p_info; +// See Note [Nonmoving write barrier in Perform{Put,Take}]. #define PerformPut(stack,lval) \ W_ sp; \ sp = StgStack_sp(stack) + WDS(3); \ + IF_NONMOVING_WRITE_BARRIER_ENABLED { \ + ccall updateRemembSetPushClosure_(BaseReg "ptr", W_[sp - WDS(1)] "ptr"); \ + ccall updateRemembSetPushClosure_(BaseReg "ptr", W_[sp - WDS(2)] "ptr"); \ + } \ StgStack_sp(stack) = sp; \ lval = W_[sp - WDS(1)]; diff --git a/rts/sm/NonMoving.c b/rts/sm/NonMoving.c index 99fd9c1ece..5971cbac20 100644 --- a/rts/sm/NonMoving.c +++ b/rts/sm/NonMoving.c @@ -229,6 +229,10 @@ Mutex concurrent_coll_finished_lock; * - Note [StgStack dirtiness flags and concurrent marking] (TSO.h) describes * the protocol for concurrent marking of stacks. * + * - Note [Nonmoving write barrier in Perform{Take,Put}] (PrimOps.cmm) describes + * a tricky barrier necessary when resuming threads blocked on MVar + * operations. + * * - Note [Static objects under the nonmoving collector] (Storage.c) describes * treatment of static objects. * |