[Git][ghc/ghc][wip/T19146] 2 commits: rts: Don't sanity-check StgTSO.global_link

[Ben Gamari (@bgamari)]

Ben Gamari pushed to branch wip/T19146 at Glasgow Haskell Compiler / GHC


Commits:
b7999e92 by Ben Gamari at 2023-05-03T09:10:00-04:00
rts: Don't sanity-check StgTSO.global_link

See Note [Avoid dangling global_link pointers].

Fixes #19146.

- - - - -
c69e7e6d by Ben Gamari at 2023-05-03T09:10:00-04:00
rts: Introduce printGlobalThreads

- - - - -


3 changed files:

- rts/Threads.c
- rts/Threads.h
- rts/sm/Sanity.c


Changes:

=====================================
rts/Threads.c
=====================================
@@ -1008,6 +1008,20 @@ printAllThreads(void)
   }
 }
 
+void
+printGlobalThreads(void)
+{
+  for (uint32_t g = 0; g < RtsFlags.GcFlags.generations; g++) {
+    debugBelch("\ngen %d\n", g);
+    for (StgTSO *t = generations[g].threads; t != END_TSO_QUEUE; t = t->global_link) {
+      debugBelch("thread %p (id=%lu)\n", t, t->id);
+    }
+    for (StgTSO *t = generations[g].old_threads; t != END_TSO_QUEUE; t = t->global_link) {
+      debugBelch("thread %p (id=%lu) (old)\n", t, t->id);
+    }
+  }
+}
+
 // useful from gdb
 void
 printThreadQueue(StgTSO *t)


=====================================
rts/Threads.h
=====================================
@@ -46,6 +46,7 @@ bool performTryPutMVar(Capability *cap, StgMVar *mvar, StgClosure *value);
 void printThreadBlockage (StgTSO *tso);
 void printThreadStatus (StgTSO *t);
 void printAllThreads (void);
+void printGlobalThreads(void);
 void printThreadQueue (StgTSO *t);
 #endif
 


=====================================
rts/sm/Sanity.c
=====================================
@@ -737,6 +737,27 @@ checkSTACK (StgStack *stack)
     checkStackChunk(sp, stack_end);
 }
 
+/*
+ * Note [Sanity-checking global_link]
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * TSOs are a bit odd in that they have a global_link pointer field
+ * which is not scavenged by the GC. This field is used to track the
+ * generations[_].[old_]threads lists and is ultimately updated by
+ * MarkWeak.c:tidyThreadList.
+
+ * Typically the fact that this field is not scavenged is fine as all reachable
+ * TSOs on the heap are guaranteed to be on some generation's thread list and
+ * therefore will be scavenged by tidyThreadList. However, the sanity checker
+ * poses a bit of a challenge here as it walks heap blocks directly and
+ * therefore may encounter TSOs which aren't reachable via the heap.
+ * For this reason, checkTSO does not check global_link. Instead, we only do
+ * so in checkGlobalTSOList, which by definition will only look at
+ * threads which are reachable via a thread list (and therefore must have won
+ * the forwarding-pointer race).
+ *
+ * See #19146.
+ */
+
 void
 checkTSO(StgTSO *tso)
 {
@@ -761,9 +782,11 @@ checkTSO(StgTSO *tso)
     ASSERT(LOOKS_LIKE_CLOSURE_PTR(tso->bq));
     ASSERT(LOOKS_LIKE_CLOSURE_PTR(tso->blocked_exceptions));
     ASSERT(LOOKS_LIKE_CLOSURE_PTR(tso->stackobj));
-    ASSERT(LOOKS_LIKE_CLOSURE_PTR(tso->global_link) &&
-            (tso->global_link == END_TSO_QUEUE ||
-             get_itbl((StgClosure*)tso->global_link)->type == TSO));
+
+    // This assertion is sadly not viable. See Note [Sanity-checking global_link].
+    //ASSERT(LOOKS_LIKE_CLOSURE_PTR(tso->global_link) &&
+    //        (tso->global_link == END_TSO_QUEUE ||
+    //         get_itbl((StgClosure*)tso->global_link)->type == TSO));
 
     if (tso->label) {
         ASSERT(LOOKS_LIKE_CLOSURE_PTR(tso->label));



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/a786945f3b976bcb451476f8b1710c4cc313fa4d...c69e7e6d86c7528d6cb321bb24be52633560acfb

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/a786945f3b976bcb451476f8b1710c4cc313fa4d...c69e7e6d86c7528d6cb321bb24be52633560acfb
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