[Git][ghc/ghc][wip/T22264-9.2] 2 commits: nonmoving: Fix handling of weak pointers
Ben Gamari (@bgamari)
gitlab at gitlab.haskell.org
Fri Oct 21 17:27:28 UTC 2022
Ben Gamari pushed to branch wip/T22264-9.2 at Glasgow Haskell Compiler / GHC
Commits:
29922c20 by Ben Gamari at 2022-10-21T13:25:33-04:00
nonmoving: Fix handling of weak pointers
This fixes an interaction between aging and weak pointer handling which
prevented the finalization of some weak pointers. In particular, weak
pointers could have their keys incorrectly marked by the preparatory
collector, preventing their finalization by the subsequent concurrent
collection.
While in the area, we also significantly improve the assertions
regarding weak pointers.
Fixes #22327.
- - - - -
5674bd0a by Ben Gamari at 2022-10-21T13:26:14-04:00
nonmoving: Sanity check mutable list
Assert that entries in the nonmoving generation's generational
remembered set (a.k.a. mutable list) live in nonmoving generation.
- - - - -
4 changed files:
- rts/sm/MarkWeak.c
- rts/sm/NonMoving.c
- rts/sm/NonMovingMark.c
- rts/sm/NonMovingSweep.c
Changes:
=====================================
rts/sm/MarkWeak.c
=====================================
@@ -82,6 +82,30 @@ static bool tidyWeakList (generation *gen);
static bool resurrectUnreachableThreads (generation *gen, StgTSO **resurrected_threads);
static void tidyThreadList (generation *gen);
+/*
+ * Note [Weak pointer processing and the non-moving GC]
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * When using the non-moving GC we defer weak pointer processing
+ * until the concurrent marking phase as weaks in the non-moving heap may be
+ * keyed on objects living in the non-moving generation. To accomplish this
+ * initWeakForGC keeps all weak pointers on oldest_gen->weak_ptr_list, where
+ * nonmovingCollect will find them. From there they will be moved to
+ * nonmoving_old_weak_ptr_list. During the mark loop we will move weaks with
+ * reachable keys to nonmoving_weak_ptr_list. At the end of concurrent marking
+ * we tidy the weak list (in nonmovingTidyWeakList) and perform another set of
+ * marking as necessary, just as is done in tidyWeakList.
+ *
+ * Note that this treatment takes advantage of the fact that we usually need
+ * not worry about Weak#s living in the non-moving heap but being keyed on an
+ * object in the moving heap since the Weak# must be strictly older than the
+ * key. Such objects would otherwise pose a problem since the non-moving
+ * collector would be unable to safely determine the liveness of the key.
+ * In the rare case that we *do* see such a key (e.g. in the case of a
+ * pinned ByteArray# living in a partially-filled accumulator block)
+ * the nonmoving collector assumes that it is live.
+ *
+ */
+
/*
* Prepare the weak object lists for GC. Specifically, reset weak_stage
* and move all generations' `weak_ptr_list`s to `old_weak_ptr_list`.
@@ -91,9 +115,13 @@ static void tidyThreadList (generation *gen);
void
initWeakForGC(void)
{
- uint32_t g;
+ uint32_t oldest = N;
+ if (RtsFlags.GcFlags.useNonmoving && N == oldest_gen->no) {
+ // See Note [Weak pointer processing and the non-moving GC].
+ oldest = oldest_gen->no - 1;
+ }
- for (g = 0; g <= N; g++) {
+ for (uint32_t g = 0; g <= oldest; g++) {
generation *gen = &generations[g];
gen->old_weak_ptr_list = gen->weak_ptr_list;
gen->weak_ptr_list = NULL;
@@ -265,6 +293,12 @@ static bool resurrectUnreachableThreads (generation *gen, StgTSO **resurrected_t
*/
static bool tidyWeakList(generation *gen)
{
+ if (RtsFlags.GcFlags.useNonmoving && gen == oldest_gen) {
+ // See Note [Weak pointer processing and the non-moving GC].
+ ASSERT(gen->old_weak_ptr_list == NULL);
+ return false;
+ }
+
StgWeak *w, **last_w, *next_w;
const StgInfoTable *info;
StgClosure *new;
=====================================
rts/sm/NonMoving.c
=====================================
@@ -244,6 +244,9 @@ Mutex concurrent_coll_finished_lock;
* how we use the DIRTY flags associated with MUT_VARs and TVARs to improve
* barrier efficiency.
*
+ * - Note [Weak pointer processing and the non-moving GC] (MarkWeak.c) describes
+ * how weak pointers are handled when the non-moving GC is in use.
+ *
* [ueno 2016]:
* Katsuhiro Ueno and Atsushi Ohori. 2016. A fully concurrent garbage
* collector for functional programs on multicore processors. SIGPLAN Not. 51,
@@ -282,8 +285,8 @@ Mutex concurrent_coll_finished_lock;
* was (unsurprisingly) also found to result in significant amounts of
* unnecessary copying.
*
- * Consequently, we now allow aging. Aging allows the preparatory GC leading up
- * to a major collection to evacuate some objects into the young generation.
+ * Consequently, we now allow "aging", allows the preparatory GC leading up
+ * to a major collection to evacuate objects into the young generation.
* However, this introduces the following tricky case that might arise after
* we have finished the preparatory GC:
*
@@ -292,6 +295,7 @@ Mutex concurrent_coll_finished_lock;
* ┆
* B ←────────────── A ←─────────────── root
* │ ┆ ↖─────────────── gen1 mut_list
+ * │ ┆
* ╰───────────────→ C
* ┆
*
@@ -332,6 +336,7 @@ Mutex concurrent_coll_finished_lock;
* The implementation details of this are described in Note [Non-moving GC:
* Marking evacuated objects] in Evac.c.
*
+ *
* Note [Deadlock detection under the non-moving collector]
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* In GHC the garbage collector is responsible for identifying deadlocked
=====================================
rts/sm/NonMovingMark.c
=====================================
@@ -1842,7 +1842,10 @@ bool nonmovingTidyWeaks (struct MarkQueue_ *queue)
// Otherwise it's a live weak
ASSERT(w->header.info == &stg_WEAK_info);
- if (nonmovingIsNowAlive(w->key)) {
+ // See Note [Weak pointer processing and the non-moving GC] in
+ // MarkWeak.c
+ bool key_in_nonmoving = Bdescr((StgPtr) w->key)->flags & BF_NONMOVING;
+ if (!key_in_nonmoving || nonmovingIsNowAlive(w->key)) {
nonmovingMarkLiveWeak(queue, w);
did_work = true;
=====================================
rts/sm/NonMovingSweep.c
=====================================
@@ -286,6 +286,7 @@ void nonmovingSweepMutLists()
for (bdescr *bd = old_mut_list; bd; bd = bd->link) {
for (StgPtr p = bd->start; p < bd->free; p++) {
StgClosure **q = (StgClosure**)p;
+ ASSERT(Bdescr(*q)->gen == oldest_gen);
if (nonmovingIsAlive(*q) && !is_closure_clean(*q)) {
recordMutableCap(*q, cap, oldest_gen->no);
}
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/a6fe33fa9b4503513bbaca6875dbb86f6671153e...5674bd0a899b9406e2a9349afb54244002e79611
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/a6fe33fa9b4503513bbaca6875dbb86f6671153e...5674bd0a899b9406e2a9349afb54244002e79611
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