diff --git a/include/jemalloc/internal/jemalloc_internal_inlines_a.h b/include/jemalloc/internal/jemalloc_internal_inlines_a.h
index 854fb1e2..24ea4162 100644
--- a/include/jemalloc/internal/jemalloc_internal_inlines_a.h
+++ b/include/jemalloc/internal/jemalloc_internal_inlines_a.h
@@ -146,7 +146,10 @@ tcache_get(tsd_t *tsd) {
 }
 
 static inline void
-pre_reentrancy(tsd_t *tsd) {
+pre_reentrancy(tsd_t *tsd, arena_t *arena) {
+	/* arena is the current context.  Reentry from a0 is not allowed. */
+	assert(arena != arena_get(tsd_tsdn(tsd), 0, false));
+
 	bool fast = tsd_fast(tsd);
 	++*tsd_reentrancy_levelp_get(tsd);
 	if (fast) {
diff --git a/src/arena.c b/src/arena.c
index 4e3bd6c1..b2830866 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -2051,7 +2051,7 @@ arena_new(tsdn_t *tsdn, unsigned ind, extent_hooks_t *extent_hooks) {
 		 * is done enough that we should have tsd.
 		 */
 		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		pre_reentrancy(tsdn_tsd(tsdn), arena);
 		if (hooks_arena_new_hook) {
 			hooks_arena_new_hook();
 		}
diff --git a/src/background_thread.c b/src/background_thread.c
index ab076fee..cc5100d1 100644
--- a/src/background_thread.c
+++ b/src/background_thread.c
@@ -316,7 +316,7 @@ background_threads_disable_single(tsd_t *tsd, background_thread_info_t *info) {
 		    &background_thread_lock);
 	}
 
-	pre_reentrancy(tsd);
+	pre_reentrancy(tsd, NULL);
 	malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
 	bool has_thread;
 	assert(info->state != background_thread_paused);
@@ -408,7 +408,7 @@ label_restart:
 		 */
 		malloc_mutex_unlock(tsd_tsdn(tsd), &background_thread_lock);
 
-		pre_reentrancy(tsd);
+		pre_reentrancy(tsd, NULL);
 		int err = background_thread_create_signals_masked(&info->thread,
 		    NULL, background_thread_entry, (void *)(uintptr_t)i);
 		post_reentrancy(tsd);
@@ -557,7 +557,7 @@ background_thread_create(tsd_t *tsd, unsigned arena_ind) {
 		return false;
 	}
 
-	pre_reentrancy(tsd);
+	pre_reentrancy(tsd, NULL);
 	/*
 	 * To avoid complications (besides reentrancy), create internal
 	 * background threads with the underlying pthread_create.
diff --git a/src/base.c b/src/base.c
index 22c94338..97078b13 100644
--- a/src/base.c
+++ b/src/base.c
@@ -25,11 +25,12 @@ base_map(tsdn_t *tsdn, extent_hooks_t *extent_hooks, unsigned ind, size_t size)
 	if (extent_hooks == &extent_hooks_default) {
 		addr = extent_alloc_mmap(NULL, size, PAGE, &zero, &commit);
 	} else {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		/* No arena context as we are creating new arenas. */
+		tsd_t *tsd = tsdn_null(tsdn) ? tsd_fetch() : tsdn_tsd(tsdn);
+		pre_reentrancy(tsd, NULL);
 		addr = extent_hooks->alloc(extent_hooks, NULL, size, PAGE,
 		    &zero, &commit, ind);
-		post_reentrancy(tsdn_tsd(tsdn));
+		post_reentrancy(tsd);
 	}
 
 	return addr;
@@ -64,8 +65,8 @@ base_unmap(tsdn_t *tsdn, extent_hooks_t *extent_hooks, unsigned ind, void *addr,
 		/* Nothing worked.  This should never happen. */
 		not_reached();
 	} else {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		tsd_t *tsd = tsdn_null(tsdn) ? tsd_fetch() : tsdn_tsd(tsdn);
+		pre_reentrancy(tsd, NULL);
 		if (extent_hooks->dalloc != NULL &&
 		    !extent_hooks->dalloc(extent_hooks, addr, size, true,
 		    ind)) {
@@ -88,7 +89,7 @@ base_unmap(tsdn_t *tsdn, extent_hooks_t *extent_hooks, unsigned ind, void *addr,
 		}
 		/* Nothing worked.  That's the application's problem. */
 	label_done:
-		post_reentrancy(tsdn_tsd(tsdn));
+		post_reentrancy(tsd);
 		return;
 	}
 }
diff --git a/src/extent.c b/src/extent.c
index 4b66dd88..fa45c84d 100644
--- a/src/extent.c
+++ b/src/extent.c
@@ -1025,6 +1025,18 @@ extent_alloc_default(extent_hooks_t *extent_hooks, void *new_addr, size_t size,
 	    alignment, zero, commit);
 }
 
+static void
+extent_hook_pre_reentrancy(tsdn_t *tsdn, arena_t *arena) {
+	tsd_t *tsd = tsdn_null(tsdn) ? tsd_fetch() : tsdn_tsd(tsdn);
+	pre_reentrancy(tsd, arena);
+}
+
+static void
+extent_hook_post_reentrancy(tsdn_t *tsdn) {
+	tsd_t *tsd = tsdn_null(tsdn) ? tsd_fetch() : tsdn_tsd(tsdn);
+	post_reentrancy(tsd);
+}
+
 /*
  * If virtual memory is retained, create increasingly larger extents from which
  * to split requested extents in order to limit the total number of disjoint
@@ -1073,12 +1085,11 @@ extent_grow_retained(tsdn_t *tsdn, arena_t *arena,
 		    &zeroed, &committed, (dss_prec_t)atomic_load_u(
 		    &arena->dss_prec, ATOMIC_RELAXED));
 	} else {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 		ptr = (*r_extent_hooks)->alloc(*r_extent_hooks, NULL,
 		    alloc_size, PAGE, &zeroed, &committed,
 		    arena_ind_get(arena));
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 
 	extent_init(extent, arena, ptr, alloc_size, false, NSIZES,
@@ -1250,11 +1261,10 @@ extent_alloc_wrapper_hard(tsdn_t *tsdn, arena_t *arena,
 		addr = extent_alloc_default_impl(tsdn, arena, new_addr, esize,
 		    alignment, zero, commit);
 	} else {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 		addr = (*r_extent_hooks)->alloc(*r_extent_hooks, new_addr,
 		    esize, alignment, zero, commit, arena_ind_get(arena));
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 	if (addr == NULL) {
 		extent_dalloc(tsdn, arena, extent);
@@ -1492,13 +1502,12 @@ extent_dalloc_wrapper_try(tsdn_t *tsdn, arena_t *arena,
 		err = extent_dalloc_default_impl(extent_base_get(extent),
 		    extent_size_get(extent));
 	} else {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 		err = ((*r_extent_hooks)->dalloc == NULL ||
 		    (*r_extent_hooks)->dalloc(*r_extent_hooks,
 		    extent_base_get(extent), extent_size_get(extent),
 		    extent_committed_get(extent), arena_ind_get(arena)));
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 
 	if (!err) {
@@ -1525,8 +1534,7 @@ extent_dalloc_wrapper(tsdn_t *tsdn, arena_t *arena,
 
 	extent_reregister(tsdn, extent);
 	if (*r_extent_hooks != &extent_hooks_default) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 	}
 	/* Try to decommit; purge if that fails. */
 	bool zeroed;
@@ -1550,7 +1558,7 @@ extent_dalloc_wrapper(tsdn_t *tsdn, arena_t *arena,
 		zeroed = false;
 	}
 	if (*r_extent_hooks != &extent_hooks_default) {
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 	extent_zeroed_set(extent, zeroed);
 
@@ -1595,12 +1603,11 @@ extent_destroy_wrapper(tsdn_t *tsdn, arena_t *arena,
 		extent_destroy_default_impl(extent_base_get(extent),
 		    extent_size_get(extent));
 	} else if ((*r_extent_hooks)->destroy != NULL) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 		(*r_extent_hooks)->destroy(*r_extent_hooks,
 		    extent_base_get(extent), extent_size_get(extent),
 		    extent_committed_get(extent), arena_ind_get(arena));
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 
 	extent_dalloc(tsdn, arena, extent);
@@ -1622,14 +1629,13 @@ extent_commit_impl(tsdn_t *tsdn, arena_t *arena,
 
 	extent_hooks_assure_initialized(arena, r_extent_hooks);
 	if (*r_extent_hooks != &extent_hooks_default) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 	}
 	bool err = ((*r_extent_hooks)->commit == NULL ||
 	    (*r_extent_hooks)->commit(*r_extent_hooks, extent_base_get(extent),
 	    extent_size_get(extent), offset, length, arena_ind_get(arena)));
 	if (*r_extent_hooks != &extent_hooks_default) {
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 	extent_committed_set(extent, extent_committed_get(extent) || !err);
 	return err;
@@ -1660,15 +1666,14 @@ extent_decommit_wrapper(tsdn_t *tsdn, arena_t *arena,
 	extent_hooks_assure_initialized(arena, r_extent_hooks);
 
 	if (*r_extent_hooks != &extent_hooks_default) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 	}
 	bool err = ((*r_extent_hooks)->decommit == NULL ||
 	    (*r_extent_hooks)->decommit(*r_extent_hooks,
 	    extent_base_get(extent), extent_size_get(extent), offset, length,
 	    arena_ind_get(arena)));
 	if (*r_extent_hooks != &extent_hooks_default) {
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 	extent_committed_set(extent, extent_committed_get(extent) && err);
 	return err;
@@ -1700,16 +1705,14 @@ extent_purge_lazy_impl(tsdn_t *tsdn, arena_t *arena,
 	if ((*r_extent_hooks)->purge_lazy == NULL) {
 		return true;
 	}
-
 	if (*r_extent_hooks != &extent_hooks_default) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 	}
 	bool err = (*r_extent_hooks)->purge_lazy(*r_extent_hooks,
 	    extent_base_get(extent), extent_size_get(extent), offset, length,
 	    arena_ind_get(arena));
 	if (*r_extent_hooks != &extent_hooks_default) {
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 
 	return err;
@@ -1750,14 +1753,13 @@ extent_purge_forced_impl(tsdn_t *tsdn, arena_t *arena,
 		return true;
 	}
 	if (*r_extent_hooks != &extent_hooks_default) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 	}
 	bool err = (*r_extent_hooks)->purge_forced(*r_extent_hooks,
 	    extent_base_get(extent), extent_size_get(extent), offset, length,
 	    arena_ind_get(arena));
 	if (*r_extent_hooks != &extent_hooks_default) {
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 	return err;
 }
@@ -1829,14 +1831,13 @@ extent_split_impl(tsdn_t *tsdn, arena_t *arena,
 	extent_lock2(tsdn, extent, trail);
 
 	if (*r_extent_hooks != &extent_hooks_default) {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 	}
 	bool err = (*r_extent_hooks)->split(*r_extent_hooks, extent_base_get(extent),
 	    size_a + size_b, size_a, size_b, extent_committed_get(extent),
 	    arena_ind_get(arena));
 	if (*r_extent_hooks != &extent_hooks_default) {
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 	if (err) {
 		goto label_error_c;
@@ -1908,13 +1909,12 @@ extent_merge_impl(tsdn_t *tsdn, arena_t *arena,
 		err = extent_merge_default_impl(extent_base_get(a),
 		    extent_base_get(b));
 	} else {
-		assert(!tsdn_null(tsdn));
-		pre_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_pre_reentrancy(tsdn, arena);
 		err = (*r_extent_hooks)->merge(*r_extent_hooks,
 		    extent_base_get(a), extent_size_get(a), extent_base_get(b),
 		    extent_size_get(b), extent_committed_get(a),
 		    arena_ind_get(arena));
-		post_reentrancy(tsdn_tsd(tsdn));
+		extent_hook_post_reentrancy(tsdn);
 	}
 
 	if (err) {
diff --git a/src/jemalloc.c b/src/jemalloc.c
index aa79633c..7bf23105 100644
--- a/src/jemalloc.c
+++ b/src/jemalloc.c
@@ -1476,7 +1476,7 @@ malloc_init_hard(void) {
 
 	malloc_mutex_lock(tsd_tsdn(tsd), &init_lock);
 	/* Set reentrancy level to 1 during init. */
-	pre_reentrancy(tsd);
+	pre_reentrancy(tsd, NULL);
 	/* Initialize narenas before prof_boot2 (for allocation). */
 	if (malloc_init_narenas() || background_thread_boot1(tsd_tsdn(tsd))) {
 		UNLOCK_RETURN(tsd_tsdn(tsd), true, true)
diff --git a/src/prof.c b/src/prof.c
index 61dfa2ce..975722c4 100644
--- a/src/prof.c
+++ b/src/prof.c
@@ -1633,7 +1633,7 @@ prof_dump(tsd_t *tsd, bool propagate_err, const char *filename,
 		return true;
 	}
 
-	pre_reentrancy(tsd);
+	pre_reentrancy(tsd, NULL);
 	malloc_mutex_lock(tsd_tsdn(tsd), &prof_dump_mtx);
 
 	prof_gctx_tree_t gctxs;