Move background thread state operations into background thread modules.

Move operations touch background thread's lifecycle and states back into
its own module.

include/jemalloc/internal/background_thread.h

@@ -76,6 +76,25 @@ bool background_thread_stats_read(
     tsdn_t *tsdn, background_thread_stats_t *stats);
 void background_thread_ctl_init(tsdn_t *tsdn);
 
+/*
+ * Arena-reset lifecycle bracket (owned by the bg-thread module so the
+ * started<->paused state machine is mutated only inside the module).  begin()
+ * acquires the global background_thread_lock and RETURNS WITH IT HELD;
+ * finish() releases it.  The lock intentionally spans the whole arena-reset
+ * body across the two calls.  Pass arena_ind (not an arena_t *): the destroy
+ * path frees the arena before calling finish().
+ */
+void background_thread_arena_reset_begin(tsd_t *tsd, unsigned arena_ind);
+void background_thread_arena_reset_finish(tsd_t *tsd, unsigned arena_ind);
+
+/*
+ * Serialize a rare admin operation against the background thread by holding the
+ * per-arena info mutex (have_background_thread-gated).  Used by
+ * arena_set_extent_hooks to fence pa_shard_disable_hpa.
+ */
+void background_thread_serialize_lock(tsd_t *tsd, unsigned arena_ind);
+void background_thread_serialize_unlock(tsd_t *tsd, unsigned arena_ind);
+
 #ifdef JEMALLOC_PTHREAD_CREATE_WRAPPER
 extern int pthread_create_wrapper(pthread_t *__restrict, const pthread_attr_t *,
     void *(*)(void *), void *__restrict);

include/jemalloc/internal/background_thread_inlines.h

@@ -22,12 +22,6 @@ background_thread_enabled_set(tsdn_t *tsdn, bool state) {
 	background_thread_enabled_set_impl(state);
 }
 
-JEMALLOC_ALWAYS_INLINE background_thread_info_t *
-arena_background_thread_info_get(arena_t *arena) {
-	unsigned arena_ind = arena_ind_get(arena);
-	return &background_thread_info[arena_ind % max_background_threads];
-}
-
 JEMALLOC_ALWAYS_INLINE background_thread_info_t *
 background_thread_info_get(size_t ind) {
 	return &background_thread_info[ind % max_background_threads];

src/arena.c

@@ -1427,17 +1427,17 @@ arena_get_ehooks(const arena_t *arena) {
 extent_hooks_t *
 arena_set_extent_hooks(
     tsd_t *tsd, arena_t *arena, extent_hooks_t *extent_hooks) {
-	background_thread_info_t *info;
+	/*
-	if (have_background_thread) {
+	 * Serialize disabling the HPA against the background thread via the
-		info = arena_background_thread_info_get(arena);
+	 * module-owned bracket (have_background_thread-gated internally) rather
-		malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
+	 * than reaching into info->mtx directly.
-	}
+	 */
+	unsigned arena_ind = arena_ind_get(arena);
+	background_thread_serialize_lock(tsd, arena_ind);
 	/* No using the HPA now that we have the custom hooks. */
 	pa_shard_disable_hpa(tsd_tsdn(tsd), &arena->pa_shard);
 	extent_hooks_t *ret = base_extent_hooks_set(arena->base, extent_hooks);
-	if (have_background_thread) {
+	background_thread_serialize_unlock(tsd, arena_ind);
-		malloc_mutex_unlock(tsd_tsdn(tsd), &info->mtx);
-	}
 
 	return ret;
 }

src/background_thread.c

@@ -34,6 +34,72 @@ size_t     max_background_threads;
 /* Thread info per-index. */
 background_thread_info_t *background_thread_info;
 
+/******************************************************************************/
+/*
+ * Config-independent lifecycle/state-ownership helpers.  Defined
+ * unconditionally and gated at runtime on have_background_thread, so callers in
+ * ctl.c / arena.c never touch background_thread_lock or info->state directly;
+ * they compile to runtime no-ops when !have_background_thread.
+ */
+
+void
+background_thread_arena_reset_begin(tsd_t *tsd, unsigned arena_ind) {
+	/* Temporarily disable the background thread during arena reset. */
+	if (have_background_thread) {
+		/*
+		 * Hold background_thread_lock across the whole arena-reset
+		 * body (acquired here, released in _finish) so a concurrent
+		 * background_threads_enable() cannot start a background
+		 * thread mid-reset.  This must happen whenever the feature
+		 * is compiled in (have_background_thread), even when the
+		 * thread is not currently enabled; the state transition
+		 * below is separately gated on background_thread_enabled().
+		 */
+		malloc_mutex_lock(tsd_tsdn(tsd), &background_thread_lock);
+		if (background_thread_enabled()) {
+			background_thread_info_t *info =
+			    background_thread_info_get(arena_ind);
+			assert(info->state == background_thread_started);
+			malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
+			info->state = background_thread_paused;
+			malloc_mutex_unlock(tsd_tsdn(tsd), &info->mtx);
+		}
+	}
+}
+
+void
+background_thread_arena_reset_finish(tsd_t *tsd, unsigned arena_ind) {
+	if (have_background_thread) {
+		if (background_thread_enabled()) {
+			background_thread_info_t *info =
+			    background_thread_info_get(arena_ind);
+			assert(info->state == background_thread_paused);
+			malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
+			info->state = background_thread_started;
+			malloc_mutex_unlock(tsd_tsdn(tsd), &info->mtx);
+		}
+		malloc_mutex_unlock(tsd_tsdn(tsd), &background_thread_lock);
+	}
+}
+
+void
+background_thread_serialize_lock(tsd_t *tsd, unsigned arena_ind) {
+	if (have_background_thread) {
+		background_thread_info_t *info =
+		    background_thread_info_get(arena_ind);
+		malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
+	}
+}
+
+void
+background_thread_serialize_unlock(tsd_t *tsd, unsigned arena_ind) {
+	if (have_background_thread) {
+		background_thread_info_t *info =
+		    background_thread_info_get(arena_ind);
+		malloc_mutex_unlock(tsd_tsdn(tsd), &info->mtx);
+	}
+}
+
 /******************************************************************************/
 
 #ifdef JEMALLOC_PTHREAD_CREATE_WRAPPER

src/ctl.c

@@ -2798,37 +2798,6 @@ arena_i_reset_destroy_helper(tsd_t *tsd, const size_t *mib, size_t miblen,
 	return ret;
 }
 
-static void
-arena_reset_prepare_background_thread(tsd_t *tsd, unsigned arena_ind) {
-	/* Temporarily disable the background thread during arena reset. */
-	if (have_background_thread) {
-		malloc_mutex_lock(tsd_tsdn(tsd), &background_thread_lock);
-		if (background_thread_enabled()) {
-			background_thread_info_t *info =
-			    background_thread_info_get(arena_ind);
-			assert(info->state == background_thread_started);
-			malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
-			info->state = background_thread_paused;
-			malloc_mutex_unlock(tsd_tsdn(tsd), &info->mtx);
-		}
-	}
-}
-
-static void
-arena_reset_finish_background_thread(tsd_t *tsd, unsigned arena_ind) {
-	if (have_background_thread) {
-		if (background_thread_enabled()) {
-			background_thread_info_t *info =
-			    background_thread_info_get(arena_ind);
-			assert(info->state == background_thread_paused);
-			malloc_mutex_lock(tsd_tsdn(tsd), &info->mtx);
-			info->state = background_thread_started;
-			malloc_mutex_unlock(tsd_tsdn(tsd), &info->mtx);
-		}
-		malloc_mutex_unlock(tsd_tsdn(tsd), &background_thread_lock);
-	}
-}
-
 static int
 arena_i_reset_ctl(tsd_t *tsd, const size_t *mib, size_t miblen, void *oldp,
     size_t *oldlenp, void *newp, size_t newlen) {
@@ -2842,9 +2811,9 @@ arena_i_reset_ctl(tsd_t *tsd, const size_t *mib, size_t miblen, void *oldp,
 		return ret;
 	}
 
-	arena_reset_prepare_background_thread(tsd, arena_ind);
+	background_thread_arena_reset_begin(tsd, arena_ind);
 	arena_reset(tsd, arena);
-	arena_reset_finish_background_thread(tsd, arena_ind);
+	background_thread_arena_reset_finish(tsd, arena_ind);
 
 	return ret;
 }
@@ -2871,7 +2840,7 @@ arena_i_destroy_ctl(tsd_t *tsd, const size_t *mib, size_t miblen, void *oldp,
 		goto label_return;
 	}
 
-	arena_reset_prepare_background_thread(tsd, arena_ind);
+	background_thread_arena_reset_begin(tsd, arena_ind);
 	/* Merge stats after resetting and purging arena. */
 	arena_reset(tsd, arena);
 	arena_decay(tsd_tsdn(tsd), arena, false, true);
@@ -2885,7 +2854,7 @@ arena_i_destroy_ctl(tsd_t *tsd, const size_t *mib, size_t miblen, void *oldp,
 	/* Record arena index for later recycling via arenas.create. */
 	ql_elm_new(ctl_arena, destroyed_link);
 	ql_tail_insert(&ctl_arenas->destroyed, ctl_arena, destroyed_link);
-	arena_reset_finish_background_thread(tsd, arena_ind);
+	background_thread_arena_reset_finish(tsd, arena_ind);
 
 	assert(ret == 0);
 label_return: