erts: Fix ASSERT crash in erts_try_seize_code_write_permission

ASSERT(code_writing_process != c_p);

The assert was actually harmless and could be removed as one solution.
But I think it's better to keep it as a way to catch bugs that would
otherwise lead to horrible deadlocks if a thread "forgets" to call
erts_release_code_write_permission().

As a result erts_release_code_write_permission() must be called BEFORE
resuming the suspended process.

3 files changed, 8 insertions, 7 deletions

diff --git a/erts/emulator/beam/beam_bif_load.c b/erts/emulator/beam/beam_bif_load.c
index e0a4f86d2d..73264214ce 100644
--- a/erts/emulator/beam/beam_bif_load.c
+++ b/erts/emulator/beam/beam_bif_load.c
@@ -389,16 +389,16 @@ static void smp_code_ix_commiter(void* null)
     Process* p = commiter_state.stager;
 
     erts_commit_staging_code_ix();
+#ifdef DEBUG
+    commiter_state.stager = NULL;
+#endif
+    erts_release_code_write_permission();
     erts_smp_proc_lock(p, ERTS_PROC_LOCK_STATUS);
     if (!ERTS_PROC_IS_EXITING(p)) {
 	erts_resume(p, ERTS_PROC_LOCK_STATUS);
     }
     erts_smp_proc_unlock(p, ERTS_PROC_LOCK_STATUS);
     erts_smp_proc_dec_refc(p);
-#ifdef DEBUG
-    commiter_state.stager = NULL;
-#endif
-    erts_release_code_write_permission();
 }
 #endif /* ERTS_SMP */
 
diff --git a/erts/emulator/beam/code_ix.h b/erts/emulator/beam/code_ix.h
index e2bd0da112..3f4f9776a4 100644
--- a/erts/emulator/beam/code_ix.h
+++ b/erts/emulator/beam/code_ix.h
@@ -76,7 +76,7 @@ ErtsCodeIndex erts_active_code_ix(void);
 
 /* Return staging code ix.
  * Only used by a process performing code loading/upgrading/deleting/purging.
- * code_ix must be locked.
+ * Code write permission must be seized.
  */
 ERTS_GLB_INLINE
 ErtsCodeIndex erts_staging_code_ix(void);
@@ -84,9 +84,10 @@ ErtsCodeIndex erts_staging_code_ix(void);
 /* Try seize exclusive code write permission. Needed for code staging.
  * Main process lock (only) must be held.
  * System thread progress must not be blocked.
+ * Caller must not already hold the code write permission.
  * Caller is suspended and *must* yield if 0 is returned. 
  */
-int erts_try_seize_code_write_permission(struct process*);
+int erts_try_seize_code_write_permission(struct process* c_p);
 
 /* Release code write permission.
  * Will resume any suspended waiters.
diff --git a/erts/emulator/beam/erl_bif_trace.c b/erts/emulator/beam/erl_bif_trace.c
index 99a4394666..69105b8f27 100644
--- a/erts/emulator/beam/erl_bif_trace.c
+++ b/erts/emulator/beam/erl_bif_trace.c
@@ -386,13 +386,13 @@ static void smp_bp_finisher(void* null)
 #ifdef DEBUG
 	finish_bp.stager = NULL;
 #endif
+	erts_release_code_write_permission();
 	erts_smp_proc_lock(p, ERTS_PROC_LOCK_STATUS);
 	if (!ERTS_PROC_IS_EXITING(p)) {
 	    erts_resume(p, ERTS_PROC_LOCK_STATUS);
 	}
 	erts_smp_proc_unlock(p, ERTS_PROC_LOCK_STATUS);
 	erts_smp_proc_dec_refc(p);
-	erts_release_code_write_permission();
     }
 }
 #endif /* ERTS_SMP */