When adopting a thread, spin until GC isn't running.

src/gc.c

@@ -3100,7 +3100,7 @@ static void sweep_finalizer_list(arraylist_t *list)
 }
 
 // collector entry point and control
-static _Atomic(uint32_t) jl_gc_disable_counter = 1;
+_Atomic(uint32_t) jl_gc_disable_counter = 1;
 
 JL_DLLEXPORT int jl_gc_enable(int on)
 {
@@ -3497,7 +3497,7 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
 
     jl_task_t *ct = jl_current_task;
     jl_ptls_t ptls = ct->ptls;
-    if (jl_atomic_load_relaxed(&jl_gc_disable_counter)) {
+    if (jl_atomic_load_acquire(&jl_gc_disable_counter)) {
         size_t localbytes = jl_atomic_load_relaxed(&ptls->gc_num.allocd) + gc_num.interval;
         jl_atomic_store_relaxed(&ptls->gc_num.allocd, -(int64_t)gc_num.interval);
         static_assert(sizeof(_Atomic(uint64_t)) == sizeof(gc_num.deferred_alloc), "");
@@ -3508,11 +3508,10 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
 
     int8_t old_state = jl_atomic_load_relaxed(&ptls->gc_state);
     jl_atomic_store_release(&ptls->gc_state, JL_GC_STATE_WAITING);
-    // `jl_safepoint_start_gc()` makes sure only one thread can
-    // run the GC.
+    // `jl_safepoint_start_gc()` makes sure only one thread can run the GC.
     uint64_t t0 = jl_hrtime();
     if (!jl_safepoint_start_gc()) {
-        // Multithread only. See assertion in `safepoint.c`
+        // either another thread is running GC, or the GC got disabled just now.
         jl_gc_state_set(ptls, old_state, JL_GC_STATE_WAITING);
         return;
     }
@@ -3549,7 +3548,7 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
     gc_invoke_callbacks(jl_gc_cb_pre_gc_t,
         gc_cblist_pre_gc, (collection));
 
-    if (!jl_atomic_load_relaxed(&jl_gc_disable_counter)) {
+    if (!jl_atomic_load_acquire(&jl_gc_disable_counter)) {
         JL_LOCK_NOGC(&finalizers_lock); // all the other threads are stopped, so this does not make sense, right? otherwise, failing that, this seems like plausibly a deadlock
 #ifndef __clang_gcanalyzer__
         if (_jl_gc_collect(ptls, collection)) {

src/julia_internal.h

@@ -884,6 +884,7 @@ STATIC_INLINE int jl_addr_is_safepoint(uintptr_t addr)
     return addr >= safepoint_addr && addr < safepoint_addr + jl_page_size * 3;
 }
 extern _Atomic(uint32_t) jl_gc_running;
+extern _Atomic(uint32_t) jl_gc_disable_counter;
 // All the functions are safe to be called from within a signal handler
 // provided that the thread will not be interrupted by another asynchronous
 // signal.

src/safepoint.c

@@ -124,6 +124,14 @@ int jl_safepoint_start_gc(void)
         jl_safepoint_wait_gc();
         return 0;
     }
+    // Foreign thread adoption disables the GC and waits for it to finish, however, that may
+    // introduce a race between it and this thread checking if the GC is enabled and only
+    // then setting jl_gc_running. To avoid that, check again now that we won that race.
+    if (jl_atomic_load_acquire(&jl_gc_disable_counter)) {
+        jl_atomic_store_release(&jl_gc_running, 0);
+        uv_mutex_unlock(&safepoint_lock);
+        return 0;
+    }
     jl_safepoint_enable(1);
     jl_safepoint_enable(2);
     uv_mutex_unlock(&safepoint_lock);

src/threading.c

@@ -406,18 +406,28 @@ jl_ptls_t jl_init_threadtls(int16_t tid)
     return ptls;
 }
 
-JL_DLLEXPORT jl_gcframe_t **jl_adopt_thread(void) JL_NOTSAFEPOINT_LEAVE
-{
+JL_DLLEXPORT jl_gcframe_t **jl_adopt_thread(void)
+{
+    // `jl_init_threadtls` puts us in a GC unsafe region, so ensure GC isn't running.
+    // we can't use a normal safepoint because we don't have signal handlers yet.
+    // we also can't use jl_safepoint_wait_gc because that assumes we're in a task.
+    jl_atomic_fetch_add(&jl_gc_disable_counter, 1);
+    while (jl_atomic_load_acquire(&jl_gc_running)) {
+        jl_cpu_pause();
+    }
+    // this check is coupled with the one in `jl_safepoint_wait_gc`, where we observe if a
+    // foreign thread has asked to disable the GC, guaranteeing the order of events.
+
     // initialize this thread (assign tid, create heap, set up root task)
     jl_ptls_t ptls = jl_init_threadtls(-1);
     void *stack_lo, *stack_hi;
     jl_init_stack_limits(0, &stack_lo, &stack_hi);
 
-    (void)jl_gc_unsafe_enter(ptls);
     // warning: this changes `jl_current_task`, so be careful not to call that from this function
-    jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
+    jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi); // assumes the GC is disabled
     JL_GC_PROMISE_ROOTED(ct);
     uv_random(NULL, NULL, &ct->rngState, sizeof(ct->rngState), 0, NULL);
+    jl_atomic_fetch_add(&jl_gc_disable_counter, -1);
     return &ct->gcstack;
 }