erts: Activate staged code in a thread safe way

Activation of staged code is scheduled for a later moment when
all schedulers have done a full memory barrier. This allow
them to read active code index while executing without any
memory barriers at all.

6 files changed, 154 insertions, 57 deletions

diff --git a/erts/emulator/beam/beam_bif_load.c b/erts/emulator/beam/beam_bif_load.c
index 1227af8cbb..6df5774fdb 100644
--- a/erts/emulator/beam/beam_bif_load.c
+++ b/erts/emulator/beam/beam_bif_load.c
@@ -35,6 +35,7 @@
 #include "erl_nif.h"
 #include "erl_thr_progress.h"
 
+static Eterm staging_epilogue(Process* c_p, int, Eterm res, int);
 static void set_default_trace_pattern(Eterm module);
 static Eterm check_process_code(Process* rp, Module* modp);
 static void delete_code(Module* modp);
@@ -86,7 +87,8 @@ BIF_RETTYPE code_make_stub_module_3(BIF_ALIST_3)
     res = erts_make_stub_module(BIF_P, BIF_ARG_1, BIF_ARG_2, BIF_ARG_3);
 
     if (res == BIF_ARG_1) {
-	erts_commit_staging_code_ix();
+	erts_end_staging_code_ix();
+	erts_activate_staging_code_ix();
     }
     else {
 	erts_abort_staging_code_ix();
@@ -166,6 +168,7 @@ finish_loading_1(BIF_ALIST_1)
     Uint exceptions;
     Eterm res;
     int is_blocking = 0;
+    int do_commit = 0;
 
     if (!erts_try_lock_code_ix(BIF_P)) {
 	ERTS_BIF_YIELD1(bif_export[BIF_finish_loading_1], BIF_P, BIF_ARG_1);
@@ -272,7 +275,6 @@ finish_loading_1(BIF_ALIST_1)
 
     if (exceptions) {
 	res = exception_list(BIF_P, am_not_purged, p, exceptions);
-	erts_abort_staging_code_ix();
     } else {
 	/*
 	 * Now we can load all code. This can't fail.
@@ -299,20 +301,63 @@ finish_loading_1(BIF_ALIST_1)
 	if (exceptions) {
 	    res = exception_list(BIF_P, am_on_load, p, exceptions);
 	}
-	erts_commit_staging_code_ix();
+	do_commit = 1;
     }
 
 done:
     if (p) {
 	erts_free(ERTS_ALC_T_LOADER_TMP, p);
     }
-    if (!is_blocking) {
-	erts_unlock_code_ix();
-    } else {
-	erts_smp_thr_progress_unblock();
-	erts_smp_proc_lock(BIF_P, ERTS_PROC_LOCK_MAIN);
+    return staging_epilogue(BIF_P, do_commit, res, is_blocking);
+}
+
+static Eterm
+staging_epilogue(Process* c_p, int commit, Eterm res, int is_blocking)
+{    
+#ifdef ERTS_SMP
+    if (is_blocking || !commit)
+#endif
+    {
+	if (commit) {
+	    erts_end_staging_code_ix();
+	    erts_activate_staging_code_ix();
+	}
+	else {
+	    erts_abort_staging_code_ix();
+	}
+	if (is_blocking) {
+	    erts_smp_thr_progress_unblock();
+	    erts_smp_proc_lock(c_p, ERTS_PROC_LOCK_MAIN);
+	}
+	else {
+	    erts_unlock_code_ix();
+	}
+	return res;
     }
-    BIF_RET(res);
+#ifdef ERTS_SMP
+    else {
+	ErtsThrPrgrVal later;
+	ASSERT(is_value(res));
+
+	erts_end_staging_code_ix();
+	/*
+	 * Now we must wait for all schedulers to do a memory barrier before
+	 * we can activate and let them access the new staged code. This allows
+	 * schedulers to read active code_ix in a safe way while executing
+	 * without any memory barriers at all. 
+	 */
+    
+	later = erts_thr_progress_later(); 
+	erts_thr_progress_wakeup(c_p->scheduler_data, later);
+	erts_notify_code_ix_activation(c_p, later);
+	erts_suspend(c_p, ERTS_PROC_LOCK_MAIN, NULL);
+	/*
+	 * handle_code_ix_activation() will do the rest "later"
+	 * and resume this process in bif_return_trap() to return 'res'.  
+	 */
+	ERTS_BIF_YIELD1(&bif_return_trap_export, c_p, res);
+    }
+#endif
 }
 
 BIF_RETTYPE
@@ -407,17 +452,18 @@ BIF_RETTYPE delete_module_1(BIF_ALIST_1)
     ErtsCodeIndex code_ix;
     Module* modp;
     int is_blocking = 0;
+    int success = 0;
     Eterm res = NIL;
 
     if (is_not_atom(BIF_ARG_1)) {
-	goto badarg;
+	BIF_ERROR(BIF_P, BADARG);
     }
 
     if (!erts_try_lock_code_ix(BIF_P)) {
 	ERTS_BIF_YIELD1(bif_export[BIF_delete_module_1], BIF_P, BIF_ARG_1);
     }
-
-    do {
+retry:
+    {
 	erts_start_staging_code_ix();
 	code_ix = erts_staging_code_ix();
 	modp = erts_get_module(BIF_ARG_1, code_ix);
@@ -429,7 +475,7 @@ BIF_RETTYPE delete_module_1(BIF_ALIST_1)
 	    erts_dsprintf(dsbufp, "Module %T must be purged before loading\n",
 			  BIF_ARG_1);
 	    erts_send_error_to_logger(BIF_P->group_leader, dsbufp);
-	    res = am_badarg;
+	    ERTS_BIF_PREP_ERROR(res, BIF_P, BADARG);
 	}
 	else {
 	    if (!is_blocking) {
@@ -441,7 +487,7 @@ BIF_RETTYPE delete_module_1(BIF_ALIST_1)
 		    erts_smp_proc_unlock(BIF_P, ERTS_PROC_LOCK_MAIN);
 		    erts_smp_thr_progress_block();
 		    is_blocking = 1;
-		    continue;
+		    goto retry;
 		}
 	    }
 	    else if (modp->curr.num_breakpoints) {
@@ -450,28 +496,11 @@ BIF_RETTYPE delete_module_1(BIF_ALIST_1)
 	    }
 	    delete_code(modp);
 	    res = am_true;
+	    success = 1;
 	}
-    } while (res == NIL);
-
-    if (res == am_true) {
-	erts_commit_staging_code_ix();
-    }
-    else {
-	erts_abort_staging_code_ix();
-    }
-    if (is_blocking) {
-	erts_smp_thr_progress_unblock();
-	erts_smp_proc_lock(BIF_P, ERTS_PROC_LOCK_MAIN);
-    }
-    else {
-	erts_unlock_code_ix();
     }
 
-    if (res == am_badarg) {
-    badarg:
-	BIF_ERROR(BIF_P, BADARG);
-    }
-    BIF_RET(res);
+    return staging_epilogue(BIF_P, success, res, is_blocking);  
 }
 
 BIF_RETTYPE module_loaded_1(BIF_ALIST_1)
diff --git a/erts/emulator/beam/code_ix.c b/erts/emulator/beam/code_ix.c
index a1db003127..5565b51e7e 100644
--- a/erts/emulator/beam/code_ix.c
+++ b/erts/emulator/beam/code_ix.c
@@ -66,22 +66,25 @@ void erts_start_staging_code_ix(void)
 }
 
 
-void erts_commit_staging_code_ix(void)
+void erts_end_staging_code_ix(void)
 {
     beam_catches_end_staging(1);
     export_end_staging(1);
     module_end_staging(1);
     erts_end_staging_ranges(1);
-    {
-	ErtsCodeIndex ix;
-	export_write_lock();
-	ix = erts_staging_code_ix();
-	erts_smp_atomic32_set_nob(&the_active_code_index, ix);
-	ix = (ix + 1) % ERTS_NUM_CODE_IX;
-	erts_smp_atomic32_set_nob(&the_staging_code_index, ix);
-	export_write_unlock();
-    }
-    CIX_TRACE("commit");
+    CIX_TRACE("end");
+}
+
+void erts_activate_staging_code_ix(void)
+{
+    ErtsCodeIndex ix;
+    export_write_lock();
+    ix = erts_staging_code_ix();
+    erts_smp_atomic32_set_nob(&the_active_code_index, ix);
+    ix = (ix + 1) % ERTS_NUM_CODE_IX;
+    erts_smp_atomic32_set_nob(&the_staging_code_index, ix);
+    export_write_unlock();
+    CIX_TRACE("activate");
 }
 
 void erts_abort_staging_code_ix(void)
@@ -121,7 +124,7 @@ int erts_try_lock_code_ix(Process* c_p)
 
 /* Unlock code_ix (resume all waiters)
 */
-void erts_unlock_code_ix()
+void erts_unlock_code_ix(void)
 {
     erts_smp_mtx_lock(&the_code_ix_queue_lock);
     while (the_code_ix_queue != NULL) { /* unleash the entire herd */
diff --git a/erts/emulator/beam/code_ix.h b/erts/emulator/beam/code_ix.h
index 66543fa2a8..bac8c3365c 100644
--- a/erts/emulator/beam/code_ix.h
+++ b/erts/emulator/beam/code_ix.h
@@ -91,22 +91,27 @@ int erts_try_lock_code_ix(struct process*);
  */
 void erts_unlock_code_ix(void);
 
-/* Make the "staging area" a complete copy of the active code.
+/* Prepare the "staging area" to be a complete copy of the active code.
  * code_ix must be locked.
- * Must be followed by a call to either "commit" or "abort" before code_ix lock
- * is released.
+ * Must be followed by calls to either "end" and "activate" or "abort" before
+ * code_ix lock is released.
  */
 void erts_start_staging_code_ix(void);
 
-/* Commit the staging area and update the active code index.
- * code_ix must be locked and erts_start_staging_code_ix() called.
- * ToDo: Updating active code index should be done according to Rickard's recipe.
- *       This function might need to be split into two.
+/* End the staging.
+ * code_ix must be locked.
+ * Must be followed by a call to either "activate" or "abort"
+ * before code_ix lock is released.
+ */
+void erts_end_staging_code_ix(void);
+
+/* Set staging code index as new active code index.
+ * code_ix must be locked and "start" and "end" called.
  */
-void erts_commit_staging_code_ix(void);
+void erts_activate_staging_code_ix(void);
 
 /* Abort the staging.
- * code_ix must be locked and erts_start_staging_code_ix() called.
+ * code_ix must be locked and "start" called.
  */
 void erts_abort_staging_code_ix(void);
 
diff --git a/erts/emulator/beam/erl_init.c b/erts/emulator/beam/erl_init.c
index 2d192b3fc4..a4690891d5 100644
--- a/erts/emulator/beam/erl_init.c
+++ b/erts/emulator/beam/erl_init.c
@@ -1473,7 +1473,8 @@ erl_start(int argc, char **argv)
 
     init_shared_memory(boot_argc, boot_argv);
     load_preloaded();
-    erts_commit_staging_code_ix();
+    erts_end_staging_code_ix();
+    erts_activate_staging_code_ix();
 
     erts_initialized = 1;
 
diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
index 44a99e1f84..aaee951107 100644
--- a/erts/emulator/beam/erl_process.c
+++ b/erts/emulator/beam/erl_process.c
@@ -366,6 +366,9 @@ dbg_chk_aux_work_val(erts_aint32_t value)
 #ifdef ERTS_SMP_SCHEDULERS_NEED_TO_CHECK_CHILDREN
     valid |= ERTS_SSI_AUX_WORK_CHECK_CHILDREN;
 #endif
+#ifdef ERTS_SMP
+    valid |= ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION;
+#endif
 
     if (~valid & value)
 	erl_exit(ERTS_ABORT_EXIT,
@@ -861,7 +864,7 @@ set_aux_work_flags_wakeup_nob(ErtsSchedulerSleepInfo *ssi,
     }
 }
 
-#if 0 /* Currently not used */
+#ifdef ERTS_SMP
 
 static ERTS_INLINE void
 set_aux_work_flags_wakeup_relb(ErtsSchedulerSleepInfo *ssi,
@@ -882,7 +885,7 @@ set_aux_work_flags_wakeup_relb(ErtsSchedulerSleepInfo *ssi,
     }
 }
 
-#endif
+#endif /* ERTS_SMP */
 
 static ERTS_INLINE erts_aint32_t
 set_aux_work_flags(ErtsSchedulerSleepInfo *ssi, erts_aint32_t flgs)
@@ -1145,7 +1148,49 @@ handle_async_ready_clean(ErtsAuxWorkData *awdp,
     }
 }
 
+#endif /* ERTS_USE_ASYNC_READY_Q */
+
+#ifdef ERTS_SMP
+void
+erts_notify_code_ix_activation(Process* p, ErtsThrPrgrVal later)
+{
+    ErtsAuxWorkData* awdp = &p->scheduler_data->aux_work_data;
+    ASSERT(awdp->code_ix_activation.code_stager == NULL);
+    awdp->code_ix_activation.code_stager = p;
+    awdp->code_ix_activation.thr_prgr = later;
+    erts_smp_proc_inc_refc(p);
+    set_aux_work_flags_wakeup_relb(p->scheduler_data->ssi,
+				   ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION);
+}
+
+static erts_aint32_t
+handle_code_ix_activation(ErtsAuxWorkData *awdp, erts_aint32_t aux_work)
+{
+    Process* p;
+    if (!erts_thr_progress_has_reached(awdp->code_ix_activation.thr_prgr)) {
+	return aux_work & ~ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION;
+    }
+    unset_aux_work_flags(awdp->ssi, ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION);
+    p = awdp->code_ix_activation.code_stager;
+    ASSERT(p);
+#ifdef DEBUG
+    awdp->code_ix_activation.code_stager = NULL;
 #endif
+    erts_smp_proc_lock(p, ERTS_PROC_LOCK_STATUS);
+    if (!ERTS_PROC_IS_EXITING(p)) {
+	erts_activate_staging_code_ix();
+	erts_resume(p, ERTS_PROC_LOCK_STATUS);
+	erts_smp_proc_unlock(p, ERTS_PROC_LOCK_STATUS);
+    }
+    else {
+	erts_smp_proc_unlock(p, ERTS_PROC_LOCK_STATUS);
+	erts_abort_staging_code_ix();
+    }
+    erts_unlock_code_ix();
+    erts_smp_proc_dec_refc(p);
+    return aux_work & ~ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION;
+}
+#endif /* ERTS_SMP */
 
 static ERTS_INLINE erts_aint32_t
 handle_fix_alloc(ErtsAuxWorkData *awdp, erts_aint32_t aux_work)
@@ -1451,6 +1496,10 @@ handle_aux_work(ErtsAuxWorkData *awdp, erts_aint32_t orig_aux_work)
 		    handle_mseg_cache_check);
 #endif
 
+#ifdef ERTS_SMP
+    HANDLE_AUX_WORK(ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION,
+		    handle_code_ix_activation);
+#endif
     ERTS_DBG_CHK_AUX_WORK_VAL(aux_work);
 
     return aux_work;
diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
index c23810f15a..3f19d92fcd 100644
--- a/erts/emulator/beam/erl_process.h
+++ b/erts/emulator/beam/erl_process.h
@@ -264,6 +264,7 @@ typedef enum {
 #define ERTS_SSI_AUX_WORK_CHECK_CHILDREN	(((erts_aint32_t) 1) << 8)
 #define ERTS_SSI_AUX_WORK_SET_TMO		(((erts_aint32_t) 1) << 9)
 #define ERTS_SSI_AUX_WORK_MSEG_CACHE_CHECK	(((erts_aint32_t) 1) << 10)
+#define ERTS_SSI_AUX_WORK_CODE_IX_ACTIVATION	(((erts_aint32_t) 1) << 11)
 
 typedef struct ErtsSchedulerSleepInfo_ ErtsSchedulerSleepInfo;
 
@@ -429,6 +430,12 @@ typedef struct {
 	void *queue;
     } async_ready;
 #endif
+#ifdef ERTS_SMP
+    struct {
+	Process* code_stager;
+	ErtsThrPrgrVal thr_prgr;
+    } code_ix_activation;
+#endif
 } ErtsAuxWorkData;
 
 struct ErtsSchedulerData_ {
@@ -1110,6 +1117,9 @@ void erts_smp_notify_check_children_needed(void);
 #if ERTS_USE_ASYNC_READY_Q
 void erts_notify_check_async_ready_queue(void *);
 #endif
+#ifdef ERTS_SMP
+void erts_notify_code_ix_activation(Process* p, ErtsThrPrgrVal later);
+#endif
 void erts_schedule_misc_aux_work(int sched_id,
 				 void (*func)(void *),
 				 void *arg);