Be more careful about iterator invalidation during recursive invalida… (JuliaLang#57934)

…tion

It is possible for one MethodInstance to have a backedge to itself
(`typejoin` is a prime example). This didn't used to be much of a
problem prior to JuliaLang#57625 because we would just delete the backedges list
at the top of invalidation. However, now that we're more selective, we
need to be careful not to move backedges around while a frame higher on
the stack may be looking at it. To this end, move the compaction part of
the deletion into a separate pass and only delete (but don't move
around) backedges while a frame higher on the stack may be looking at
it.

Fixes JuliaLang#57696

Author: Keno

Compiler/test/invalidation.jl

@@ -283,3 +283,8 @@ begin take!(GLOBAL_BUFFER)
     @test isnothing(pr48932_caller_inlined(42))
     @test "42" == String(take!(GLOBAL_BUFFER))
 end
+
+# Issue #57696
+# This test checks for invalidation of recursive backedges. However, unfortunately, the original failure
+# manifestation was an unreliable segfault or an assertion failure, so we don't have a more compact test.
+@test success(`$(Base.julia_cmd()) -e 'Base.typejoin(x, ::Type) = 0; exit()'`)

src/gf.c

@@ -1831,43 +1831,133 @@ JL_DLLEXPORT void jl_invalidate_code_instance(jl_code_instance_t *replaced, size
 }
 
 static void _invalidate_backedges(jl_method_instance_t *replaced_mi, jl_code_instance_t *replaced_ci, size_t max_world, int depth) {
-    jl_array_t *backedges = replaced_mi->backedges;
-    if (backedges) {
-        // invalidate callers (if any)
+    uint8_t recursion_flags = 0;
+    jl_array_t *backedges = jl_mi_get_backedges_mutate(replaced_mi, &recursion_flags);
+    if (!backedges)
+        return;
+    // invalidate callers (if any)
+    if (!replaced_ci) {
+        // We know all backedges are deleted - clear them eagerly
+        // Clears both array and flags
         replaced_mi->backedges = NULL;
-        JL_GC_PUSH1(&backedges);
-        size_t i = 0, l = jl_array_nrows(backedges);
-        size_t ins = 0;
-        jl_code_instance_t *replaced;
-        while (i < l) {
-            jl_value_t *invokesig = NULL;
-            i = get_next_edge(backedges, i, &invokesig, &replaced);
-            JL_GC_PROMISE_ROOTED(replaced); // propagated by get_next_edge from backedges
-            if (replaced_ci) {
-                // If we're invalidating a particular codeinstance, only invalidate
-                // this backedge it actually has an edge for our codeinstance.
-                jl_svec_t *edges = jl_atomic_load_relaxed(&replaced->edges);
-                for (size_t j = 0; j < jl_svec_len(edges); ++j) {
-                    jl_value_t *edge = jl_svecref(edges, j);
-                    if (edge == (jl_value_t*)replaced_mi || edge == (jl_value_t*)replaced_ci)
-                        goto found;
-                }
-                // Keep this entry in the backedge list, but compact it
-                ins = set_next_edge(backedges, ins, invokesig, replaced);
-                continue;
-            found:;
+        jl_atomic_fetch_and_relaxed(&replaced_mi->flags, ~MI_FLAG_BACKEDGES_ALL);
+    }
+    JL_GC_PUSH1(&backedges);
+    size_t i = 0, l = jl_array_nrows(backedges);
+    size_t ins = 0;
+    jl_code_instance_t *replaced;
+    while (i < l) {
+        jl_value_t *invokesig = NULL;
+        i = get_next_edge(backedges, i, &invokesig, &replaced);
+        if (!replaced) {
+            ins = i;
+            continue;
+        }
+        JL_GC_PROMISE_ROOTED(replaced); // propagated by get_next_edge from backedges
+        if (replaced_ci) {
+            // If we're invalidating a particular codeinstance, only invalidate
+            // this backedge it actually has an edge for our codeinstance.
+            jl_svec_t *edges = jl_atomic_load_relaxed(&replaced->edges);
+            for (size_t j = 0; j < jl_svec_len(edges); ++j) {
+                jl_value_t *edge = jl_svecref(edges, j);
+                if (edge == (jl_value_t*)replaced_mi || edge == (jl_value_t*)replaced_ci)
+                    goto found;
             }
-            invalidate_code_instance(replaced, max_world, depth);
+            ins = set_next_edge(backedges, ins, invokesig, replaced);
+            continue;
+        found:;
+            ins = clear_next_edge(backedges, ins, invokesig, replaced);
+            jl_atomic_fetch_or(&replaced_mi->flags, MI_FLAG_BACKEDGES_DIRTY);
+            /* fallthrough */
+        }
+        invalidate_code_instance(replaced, max_world, depth);
+        if (replaced_ci && !replaced_mi->backedges) {
+            // Fast-path early out. If `invalidate_code_instance` invalidated
+            // the entire mi via a recursive edge, there's no point to keep
+            // iterating - they'll already have been invalidated.
+            break;
         }
-        if (replaced_ci && ins != 0) {
-            jl_array_del_end(backedges, l - ins);
-            // If we're only invalidating one ci, we don't know which ci any particular
-            // backedge was for, so we can't delete them. Put them back.
-            replaced_mi->backedges = backedges;
-            jl_gc_wb(replaced_mi, backedges);
+    }
+    if (replaced_ci)
+        jl_mi_done_backedges(replaced_mi, recursion_flags);
+    JL_GC_POP();
+}
+
+enum morespec_options {
+    morespec_unknown,
+    morespec_isnot,
+    morespec_is
+};
+
+// check if `type` is replacing `m` with an ambiguity here, given other methods in `d` that already match it
+static int is_replacing(char ambig, jl_value_t *type, jl_method_t *m, jl_method_t *const *d, size_t n, jl_value_t *isect, jl_value_t *isect2, char *morespec)
+{
+    size_t k;
+    for (k = 0; k < n; k++) {
+        jl_method_t *m2 = d[k];
+        // see if m2 also fully covered this intersection
+        if (m == m2 || !(jl_subtype(isect, m2->sig) || (isect2 && jl_subtype(isect2, m2->sig))))
+            continue;
+        if (morespec[k] == (char)morespec_unknown)
+            morespec[k] = (char)(jl_type_morespecific(m2->sig, type) ? morespec_is : morespec_isnot);
+        if (morespec[k] == (char)morespec_is)
+            // not actually shadowing this--m2 will still be better
+            return 0;
+        // if type is not more specific than m (thus now dominating it)
+        // then there is a new ambiguity here,
+        // since m2 was also a previous match over isect,
+        // see if m was previously dominant over all m2
+        // or if this was already ambiguous before
+        if (ambig != morespec_is && !jl_type_morespecific(m->sig, m2->sig)) {
+            // m and m2 were previously ambiguous over the full intersection of mi with type, and will still be ambiguous with addition of type
+            return 0;
         }
-        JL_GC_POP();
     }
+    return 1;
+}
+
+static int _invalidate_dispatch_backedges(jl_method_instance_t *mi, jl_value_t *type, jl_method_t *m,
+        jl_method_t *const *d, size_t n, int replaced_dispatch, int ambig,
+        size_t max_world, char *morespec)
+{
+    uint8_t backedge_recursion_flags = 0;
+    jl_array_t *backedges = jl_mi_get_backedges_mutate(mi, &backedge_recursion_flags);
+    if (!backedges)
+        return 0;
+    size_t ib = 0, insb = 0, nb = jl_array_nrows(backedges);
+    jl_value_t *invokeTypes;
+    jl_code_instance_t *caller;
+    int invalidated_any = 0;
+    while (mi->backedges && ib < nb) {
+        ib = get_next_edge(backedges, ib, &invokeTypes, &caller);
+        if (!caller) {
+            insb = ib;
+            continue;
+        }
+        JL_GC_PROMISE_ROOTED(caller); // propagated by get_next_edge from backedges
+        int replaced_edge;
+        if (invokeTypes) {
+            // n.b. normally we must have mi.specTypes <: invokeTypes <: m.sig (though it might not strictly hold), so we only need to check the other subtypes
+            if (jl_egal(invokeTypes, jl_get_ci_mi(caller)->def.method->sig))
+                replaced_edge = 0; // if invokeTypes == m.sig, then the only way to change this invoke is to replace the method itself
+            else
+                replaced_edge = jl_subtype(invokeTypes, type) && is_replacing(ambig, type, m, d, n, invokeTypes, NULL, morespec);
+        }
+        else {
+            replaced_edge = replaced_dispatch;
+        }
+        if (replaced_edge) {
+            invalidate_code_instance(caller, max_world, 1);
+            insb = clear_next_edge(backedges, insb, invokeTypes, caller);
+            jl_atomic_fetch_or(&mi->flags, MI_FLAG_BACKEDGES_DIRTY);
+            invalidated_any = 1;
+        }
+        else {
+            insb = set_next_edge(backedges, insb, invokeTypes, caller);
+        }
+    }
+    jl_mi_done_backedges(mi, backedge_recursion_flags);
+    return invalidated_any;
 }
 
 // invalidate cached methods that overlap this definition
@@ -1898,20 +1988,22 @@ JL_DLLEXPORT void jl_method_instance_add_backedge(jl_method_instance_t *callee,
     JL_LOCK(&callee->def.method->writelock);
     if (jl_atomic_load_relaxed(&allow_new_worlds)) {
         int found = 0;
+        jl_array_t *backedges = jl_mi_get_backedges(callee);
         // TODO: use jl_cache_type_(invokesig) like cache_method does to save memory
-        if (!callee->backedges) {
+        if (!backedges) {
             // lazy-init the backedges array
-            callee->backedges = jl_alloc_vec_any(0);
-            jl_gc_wb(callee, callee->backedges);
+            backedges = jl_alloc_vec_any(0);
+            callee->backedges = backedges;
+            jl_gc_wb(callee, backedges);
         }
         else {
-            size_t i = 0, l = jl_array_nrows(callee->backedges);
+            size_t i = 0, l = jl_array_nrows(backedges);
             for (i = 0; i < l; i++) {
                 // optimized version of while (i < l) i = get_next_edge(callee->backedges, i, &invokeTypes, &mi);
-                jl_value_t *ciedge = jl_array_ptr_ref(callee->backedges, i);
+                jl_value_t *ciedge = jl_array_ptr_ref(backedges, i);
                 if (ciedge != (jl_value_t*)caller)
                     continue;
-                jl_value_t *invokeTypes = i > 0 ? jl_array_ptr_ref(callee->backedges, i - 1) : NULL;
+                jl_value_t *invokeTypes = i > 0 ? jl_array_ptr_ref(backedges, i - 1) : NULL;
                 if (invokeTypes && jl_is_method_instance(invokeTypes))
                     invokeTypes = NULL;
                 if ((invokesig == NULL && invokeTypes == NULL) ||
@@ -1922,7 +2014,7 @@ JL_DLLEXPORT void jl_method_instance_add_backedge(jl_method_instance_t *callee,
             }
         }
         if (!found)
-            push_edge(callee->backedges, invokesig, caller);
+            push_edge(backedges, invokesig, caller);
     }
     JL_UNLOCK(&callee->def.method->writelock);
 }
@@ -2111,13 +2203,13 @@ static int erase_method_backedges(jl_typemap_entry_t *def, void *closure)
         for (i = 0; i < l; i++) {
             jl_method_instance_t *mi = (jl_method_instance_t*)jl_svecref(specializations, i);
             if ((jl_value_t*)mi != jl_nothing) {
-                mi->backedges = NULL;
+                mi->backedges = 0;
             }
         }
     }
     else {
         jl_method_instance_t *mi = (jl_method_instance_t*)specializations;
-        mi->backedges = NULL;
+        mi->backedges = 0;
     }
     JL_UNLOCK(&method->writelock);
     return 1;
@@ -2189,39 +2281,6 @@ static int jl_type_intersection2(jl_value_t *t1, jl_value_t *t2, jl_value_t **is
     return 1;
 }
 
-enum morespec_options {
-    morespec_unknown,
-    morespec_isnot,
-    morespec_is
-};
-
-// check if `type` is replacing `m` with an ambiguity here, given other methods in `d` that already match it
-static int is_replacing(char ambig, jl_value_t *type, jl_method_t *m, jl_method_t *const *d, size_t n, jl_value_t *isect, jl_value_t *isect2, char *morespec)
-{
-    size_t k;
-    for (k = 0; k < n; k++) {
-        jl_method_t *m2 = d[k];
-        // see if m2 also fully covered this intersection
-        if (m == m2 || !(jl_subtype(isect, m2->sig) || (isect2 && jl_subtype(isect2, m2->sig))))
-            continue;
-        if (morespec[k] == (char)morespec_unknown)
-            morespec[k] = (char)(jl_type_morespecific(m2->sig, type) ? morespec_is : morespec_isnot);
-        if (morespec[k] == (char)morespec_is)
-            // not actually shadowing this--m2 will still be better
-            return 0;
-        // if type is not more specific than m (thus now dominating it)
-        // then there is a new ambiguity here,
-        // since m2 was also a previous match over isect,
-        // see if m was previously dominant over all m2
-        // or if this was already ambiguous before
-        if (ambig != morespec_is && !jl_type_morespecific(m->sig, m2->sig)) {
-            // m and m2 were previously ambiguous over the full intersection of mi with type, and will still be ambiguous with addition of type
-            return 0;
-        }
-    }
-    return 1;
-}
-
 jl_typemap_entry_t *jl_method_table_add(jl_methtable_t *mt, jl_method_t *method, jl_tupletype_t *simpletype)
 {
     JL_TIMING(ADD_METHOD, ADD_METHOD);
@@ -2386,35 +2445,7 @@ void jl_method_table_activate(jl_methtable_t *mt, jl_typemap_entry_t *newentry)
                         // found that this specialization dispatch got replaced by m
                         // call invalidate_backedges(mi, max_world, "jl_method_table_insert");
                         // but ignore invoke-type edges
-                        jl_array_t *backedges = mi->backedges;
-                        if (backedges) {
-                            size_t ib = 0, insb = 0, nb = jl_array_nrows(backedges);
-                            jl_value_t *invokeTypes;
-                            jl_code_instance_t *caller;
-                            while (ib < nb) {
-                                ib = get_next_edge(backedges, ib, &invokeTypes, &caller);
-                                JL_GC_PROMISE_ROOTED(caller); // propagated by get_next_edge from backedges
-                                int replaced_edge;
-                                if (invokeTypes) {
-                                    // n.b. normally we must have mi.specTypes <: invokeTypes <: m.sig (though it might not strictly hold), so we only need to check the other subtypes
-                                    if (jl_egal(invokeTypes, jl_get_ci_mi(caller)->def.method->sig))
-                                        replaced_edge = 0; // if invokeTypes == m.sig, then the only way to change this invoke is to replace the method itself
-                                    else
-                                        replaced_edge = jl_subtype(invokeTypes, type) && is_replacing(ambig, type, m, d, n, invokeTypes, NULL, morespec);
-                                }
-                                else {
-                                    replaced_edge = replaced_dispatch;
-                                }
-                                if (replaced_edge) {
-                                    invalidate_code_instance(caller, max_world, 1);
-                                    invalidated = 1;
-                                }
-                                else {
-                                    insb = set_next_edge(backedges, insb, invokeTypes, caller);
-                                }
-                            }
-                            jl_array_del_end(backedges, nb - insb);
-                        }
+                        invalidated = _invalidate_dispatch_backedges(mi, type, m, d, n, replaced_dispatch, ambig, max_world, morespec);
                         jl_array_ptr_1d_push(oldmi, (jl_value_t*)mi);
                         if (_jl_debug_method_invalidation && invalidated) {
                             jl_array_ptr_1d_push(_jl_debug_method_invalidation, (jl_value_t*)mi);

src/julia.h

@@ -404,13 +404,16 @@ struct _jl_method_instance_t {
     } def; // pointer back to the context for this code
     jl_value_t *specTypes;  // argument types this was specialized for
     jl_svec_t *sparam_vals; // static parameter values, indexed by def.method->sig
-    jl_array_t *backedges; // list of code-instances which call this method-instance; `invoke` records (invokesig, caller) pairs
+    // list of code-instances which call this method-instance; `invoke` records (invokesig, caller) pairs
+    jl_array_t *backedges;
     _Atomic(struct _jl_code_instance_t*) cache;
     uint8_t cache_with_orig; // !cache_with_specTypes
 
     // flags for this method instance
     //   bit 0: generated by an explicit `precompile(...)`
     //   bit 1: dispatched
+    //   bit 2: The ->backedges field is currently being walked higher up the stack - entries may be deleted, but not moved
+    //   bit 3: The ->backedges field was modified and should be compacted when clearing bit 2
     _Atomic(uint8_t) flags;
 };
 #define JL_MI_FLAGS_MASK_PRECOMPILED    0x01

src/julia_internal.h

@@ -726,9 +726,29 @@ JL_DLLEXPORT void jl_resolve_definition_effects_in_ir(jl_array_t *stmts, jl_modu
 JL_DLLEXPORT int jl_maybe_add_binding_backedge(jl_binding_t *b, jl_value_t *edge, jl_method_t *in_method);
 JL_DLLEXPORT void jl_add_binding_backedge(jl_binding_t *b, jl_value_t *edge);
 
+static const uint8_t MI_FLAG_BACKEDGES_INUSE = 0b0100;
+static const uint8_t MI_FLAG_BACKEDGES_DIRTY = 0b1000;
+static const uint8_t MI_FLAG_BACKEDGES_ALL = 0b1100;
+
+STATIC_INLINE jl_array_t *jl_mi_get_backedges_mutate(jl_method_instance_t *mi JL_PROPAGATES_ROOT, uint8_t *flags) {
+    *flags = jl_atomic_load_relaxed(&mi->flags) & (MI_FLAG_BACKEDGES_ALL);
+    jl_array_t *ret = mi->backedges;
+    if (ret)
+        jl_atomic_fetch_or_relaxed(&mi->flags, MI_FLAG_BACKEDGES_INUSE);
+    return ret;
+}
+
+STATIC_INLINE jl_array_t *jl_mi_get_backedges(jl_method_instance_t *mi JL_PROPAGATES_ROOT) {
+    assert(!(jl_atomic_load_relaxed(&mi->flags) & MI_FLAG_BACKEDGES_ALL));
+    jl_array_t *ret = mi->backedges;
+    return ret;
+}
+
 int get_next_edge(jl_array_t *list, int i, jl_value_t** invokesig, jl_code_instance_t **caller) JL_NOTSAFEPOINT;
 int set_next_edge(jl_array_t *list, int i, jl_value_t *invokesig, jl_code_instance_t *caller);
+int clear_next_edge(jl_array_t *list, int i, jl_value_t *invokesig, jl_code_instance_t *caller);
 void push_edge(jl_array_t *list, jl_value_t *invokesig, jl_code_instance_t *caller);
+void jl_mi_done_backedges(jl_method_instance_t *mi JL_PROPAGATES_ROOT, uint8_t old_flags);
 
 JL_DLLEXPORT void jl_add_method_root(jl_method_t *m, jl_module_t *mod, jl_value_t* root);
 void jl_append_method_roots(jl_method_t *m, uint64_t modid, jl_array_t* roots);