diff --git a/src/gc.c b/src/gc.c
index a1d9cc9e8132fd..42d682b496e98e 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -1639,9 +1639,11 @@ void gc_sweep_wake_all(jl_ptls_t ptls, jl_gc_padded_page_stack_t *new_gc_allocd_
     if (parallel_sweep_worthwhile && !page_profile_enabled) {
         jl_atomic_store(&gc_allocd_scratch, new_gc_allocd_scratch);
         uv_mutex_lock(&gc_threads_lock);
-        for (int i = gc_first_tid; i < gc_first_tid + jl_n_markthreads; i++) {
+        int first = gc_first_parallel_collector_thread_id();
+        int last = gc_last_parallel_collector_thread_id();
+        for (int i = first; i <= last; i++) {
             jl_ptls_t ptls2 = gc_all_tls_states[i];
-            assert(ptls2 != NULL); // should be a GC thread
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
             jl_atomic_fetch_add(&ptls2->gc_sweeps_requested, 1);
         }
         uv_cond_broadcast(&gc_threads_cond);
@@ -1653,9 +1655,11 @@ void gc_sweep_wake_all(jl_ptls_t ptls, jl_gc_padded_page_stack_t *new_gc_allocd_
         // collecting a page profile.
         // wait for all to leave in order to ensure that a straggler doesn't
         // try to enter sweeping after we set `gc_allocd_scratch` below.
-        for (int i = gc_first_tid; i < gc_first_tid + jl_n_markthreads; i++) {
+        int first = gc_first_parallel_collector_thread_id();
+        int last = gc_last_parallel_collector_thread_id();
+        for (int i = first; i <= last; i++) {
             jl_ptls_t ptls2 = gc_all_tls_states[i];
-            assert(ptls2 != NULL); // should be a GC thread
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
             while (jl_atomic_load_acquire(&ptls2->gc_sweeps_requested) != 0) {
                 jl_cpu_pause();
             }
@@ -3006,10 +3010,14 @@ void gc_mark_and_steal(jl_ptls_t ptls)
     // since we know chunks will likely expand into a lot
     // of work for the mark loop
     steal : {
+        int first = gc_first_parallel_collector_thread_id();
+        int last = gc_last_parallel_collector_thread_id();
         // Try to steal chunk from random GC thread
         for (int i = 0; i < 4 * jl_n_markthreads; i++) {
-            uint32_t v = gc_first_tid + cong(UINT64_MAX, UINT64_MAX, &ptls->rngseed) % jl_n_markthreads;
-            jl_gc_markqueue_t *mq2 = &gc_all_tls_states[v]->mark_queue;
+            int v = gc_random_parallel_collector_thread_id(ptls);
+            jl_ptls_t ptls2 = gc_all_tls_states[v];
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
+            jl_gc_markqueue_t *mq2 = &ptls2->mark_queue;
             c = gc_chunkqueue_steal_from(mq2);
             if (c.cid != GC_empty_chunk) {
                 gc_mark_chunk(ptls, mq, &c);
@@ -3017,8 +3025,10 @@ void gc_mark_and_steal(jl_ptls_t ptls)
             }
         }
         // Sequentially walk GC threads to try to steal chunk
-        for (int i = gc_first_tid; i < gc_first_tid + jl_n_markthreads; i++) {
-            jl_gc_markqueue_t *mq2 = &gc_all_tls_states[i]->mark_queue;
+        for (int i = first; i <= last; i++) {
+            jl_ptls_t ptls2 = gc_all_tls_states[i];
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
+            jl_gc_markqueue_t *mq2 = &ptls2->mark_queue;
             c = gc_chunkqueue_steal_from(mq2);
             if (c.cid != GC_empty_chunk) {
                 gc_mark_chunk(ptls, mq, &c);
@@ -3035,15 +3045,19 @@ void gc_mark_and_steal(jl_ptls_t ptls)
         }
         // Try to steal pointer from random GC thread
         for (int i = 0; i < 4 * jl_n_markthreads; i++) {
-            uint32_t v = gc_first_tid + cong(UINT64_MAX, UINT64_MAX, &ptls->rngseed) % jl_n_markthreads;
-            jl_gc_markqueue_t *mq2 = &gc_all_tls_states[v]->mark_queue;
+            int v = gc_random_parallel_collector_thread_id(ptls);
+            jl_ptls_t ptls2 = gc_all_tls_states[v];
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
+            jl_gc_markqueue_t *mq2 = &ptls2->mark_queue;
             new_obj = gc_ptr_queue_steal_from(mq2);
             if (new_obj != NULL)
                 goto mark;
         }
         // Sequentially walk GC threads to try to steal pointer
-        for (int i = gc_first_tid; i < gc_first_tid + jl_n_markthreads; i++) {
-            jl_gc_markqueue_t *mq2 = &gc_all_tls_states[i]->mark_queue;
+        for (int i = first; i <= last; i++) {
+            jl_ptls_t ptls2 = gc_all_tls_states[i];
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
+            jl_gc_markqueue_t *mq2 = &ptls2->mark_queue;
             new_obj = gc_ptr_queue_steal_from(mq2);
             if (new_obj != NULL)
                 goto mark;
@@ -3103,12 +3117,13 @@ int gc_should_mark(void)
         }
         int tid = jl_atomic_load_relaxed(&gc_master_tid);
         assert(tid != -1);
+        assert(gc_all_tls_states != NULL);
         size_t work = gc_count_work_in_queue(gc_all_tls_states[tid]);
-        for (tid = gc_first_tid; tid < gc_first_tid + jl_n_markthreads; tid++) {
-            jl_ptls_t ptls2 = gc_all_tls_states[tid];
-            if (ptls2 == NULL) {
-                continue;
-            }
+        int first = gc_first_parallel_collector_thread_id();
+        int last = gc_last_parallel_collector_thread_id();
+        for (int i = first; i <= last; i++) {
+            jl_ptls_t ptls2 = gc_all_tls_states[i];
+            gc_check_ptls_of_parallel_collector_thread(ptls2);
             work += gc_count_work_in_queue(ptls2);
         }
         // if there is a lot of work left, enter the mark loop
@@ -3486,7 +3501,8 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
             jl_ptls_t ptls_dest = ptls;
             jl_gc_markqueue_t *mq_dest = mq;
             if (!single_threaded_mark) {
-                ptls_dest = gc_all_tls_states[gc_first_tid + t_i % jl_n_markthreads];
+                int dest_tid = gc_ith_parallel_collector_thread_id(t_i % jl_n_markthreads);
+                ptls_dest = gc_all_tls_states[dest_tid];
                 mq_dest = &ptls_dest->mark_queue;
             }
             if (ptls2 != NULL) {
diff --git a/src/gc.h b/src/gc.h
index bcba63a613b1f7..98d68a7bde8338 100644
--- a/src/gc.h
+++ b/src/gc.h
@@ -439,6 +439,54 @@ extern int gc_first_tid;
 extern int gc_n_threads;
 extern jl_ptls_t* gc_all_tls_states;
 
+STATIC_INLINE int gc_first_parallel_collector_thread_id(void) JL_NOTSAFEPOINT
+{
+    if (jl_n_markthreads == 0) {
+        return 0;
+    }
+    return gc_first_tid;
+}
+
+STATIC_INLINE int gc_last_parallel_collector_thread_id(void) JL_NOTSAFEPOINT
+{
+    if (jl_n_markthreads == 0) {
+        return -1;
+    }
+    return gc_first_tid + jl_n_markthreads - 1;
+}
+
+STATIC_INLINE int gc_ith_parallel_collector_thread_id(int i) JL_NOTSAFEPOINT
+{
+    assert(i >= 0 && i < jl_n_markthreads);
+    return gc_first_tid + i;
+}
+
+STATIC_INLINE int gc_is_parallel_collector_thread(int tid) JL_NOTSAFEPOINT
+{
+    return tid >= gc_first_tid && tid <= gc_last_parallel_collector_thread_id();
+}
+
+STATIC_INLINE int gc_random_parallel_collector_thread_id(jl_ptls_t ptls) JL_NOTSAFEPOINT
+{
+    assert(jl_n_markthreads > 0);
+    int v = gc_first_tid + (int)cong(jl_n_markthreads - 1, UINT64_MAX, &ptls->rngseed);
+    assert(v >= gc_first_tid && v <= gc_last_parallel_collector_thread_id());
+    return v;
+}
+
+STATIC_INLINE int gc_parallel_collector_threads_enabled(void) JL_NOTSAFEPOINT
+{
+    return jl_n_markthreads > 0;
+}
+
+STATIC_INLINE void gc_check_ptls_of_parallel_collector_thread(jl_ptls_t ptls) JL_NOTSAFEPOINT
+{
+    (void)ptls;
+    assert(gc_parallel_collector_threads_enabled());
+    assert(ptls != NULL);
+    assert(jl_atomic_load_relaxed(&ptls->gc_state) == JL_GC_PARALLEL_COLLECTOR_THREAD);
+}
+
 STATIC_INLINE bigval_t *bigval_header(jl_taggedvalue_t *o) JL_NOTSAFEPOINT
 {
     return container_of(o, bigval_t, header);
diff --git a/src/julia_threads.h b/src/julia_threads.h
index a80084a86f4b32..f6dbb7763b63e2 100644
--- a/src/julia_threads.h
+++ b/src/julia_threads.h
@@ -215,6 +215,10 @@ typedef struct _jl_tls_states_t {
 #define JL_GC_STATE_SAFE 2
     // gc_state = 2 means the thread is running unmanaged code that can be
     //              execute at the same time with the GC.
+#define JL_GC_PARALLEL_COLLECTOR_THREAD 3
+    // gc_state = 3 means the thread is a parallel collector thread (i.e. never runs Julia code)
+#define JL_GC_CONCURRENT_COLLECTOR_THREAD 4
+    // gc_state = 4 means the thread is a concurrent collector thread (background sweeper thread that never runs Julia code)
     _Atomic(int8_t) gc_state; // read from foreign threads
     // execution of certain certain impure
     // statements is prohibited from certain
@@ -343,6 +347,8 @@ void jl_sigint_safepoint(jl_ptls_t tls);
 STATIC_INLINE int8_t jl_gc_state_set(jl_ptls_t ptls, int8_t state,
                                      int8_t old_state)
 {
+    assert(old_state != JL_GC_PARALLEL_COLLECTOR_THREAD);
+    assert(old_state != JL_GC_CONCURRENT_COLLECTOR_THREAD);
     jl_atomic_store_release(&ptls->gc_state, state);
     // A safe point is required if we transition from GC-safe region to
     // non GC-safe region.
diff --git a/src/partr.c b/src/partr.c
index f8a87ac9bbc1b0..b0b4ee77a9ec5f 100644
--- a/src/partr.c
+++ b/src/partr.c
@@ -131,7 +131,7 @@ void jl_parallel_gc_threadfun(void *arg)
     jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
     JL_GC_PROMISE_ROOTED(ct);
     // wait for all threads
-    jl_gc_state_set(ptls, JL_GC_STATE_WAITING, 0);
+    jl_gc_state_set(ptls, JL_GC_PARALLEL_COLLECTOR_THREAD, 0);
     uv_barrier_wait(targ->barrier);
 
     // free the thread argument here
@@ -143,10 +143,10 @@ void jl_parallel_gc_threadfun(void *arg)
             uv_cond_wait(&gc_threads_cond, &gc_threads_lock);
         }
         uv_mutex_unlock(&gc_threads_lock);
-        if (may_mark()) {
-            gc_mark_loop_parallel(ptls, 0);
-        }
-        if (may_sweep(ptls)) { // not an else!
+        assert(jl_atomic_load_relaxed(&ptls->gc_state) == JL_GC_PARALLEL_COLLECTOR_THREAD);
+        gc_mark_loop_parallel(ptls, 0);
+        if (may_sweep(ptls)) {
+            assert(jl_atomic_load_relaxed(&ptls->gc_state) == JL_GC_PARALLEL_COLLECTOR_THREAD);
             gc_sweep_pool_parallel(ptls);
             jl_atomic_fetch_add(&ptls->gc_sweeps_requested, -1);
         }
@@ -166,13 +166,14 @@ void jl_concurrent_gc_threadfun(void *arg)
     jl_task_t *ct = jl_init_root_task(ptls, stack_lo, stack_hi);
     JL_GC_PROMISE_ROOTED(ct);
     // wait for all threads
-    jl_gc_state_set(ptls, JL_GC_STATE_WAITING, 0);
+    jl_gc_state_set(ptls, JL_GC_CONCURRENT_COLLECTOR_THREAD, 0);
     uv_barrier_wait(targ->barrier);
 
     // free the thread argument here
     free(targ);
 
     while (1) {
+        assert(jl_atomic_load_relaxed(&ptls->gc_state) == JL_GC_CONCURRENT_COLLECTOR_THREAD);
         uv_sem_wait(&gc_sweep_assists_needed);
         gc_free_pages();
     }