Fix emigrate bug in erts_port_task_schedule()

While current run-queue lock is unlocked in the call to
erts_check_emigration_need() from erts_port_task_schedule() the
port can be migrated to another run-queue by another thread.
The code in erts_port_task_schedule() needs to check if this
has occurred when returning from erts_check_emigration_need(), and
if so respect the migration decision. When this was not done,
the thread calling erts_port_task_schedule() held the wrong
run-queue lock which caused invalid updates of the port task queue.

This bug was automatically fixed by the rewrites in the branch
rickard/r16b/port-optimizations-fixes/OTP-10336 (commit
56cef897ca3ad2377e34a6ea5800a54a28cbeb6e) introduced in erts-5.10
and do not effect erts versions after that.

1 files changed, 17 insertions, 3 deletions

diff --git a/erts/emulator/beam/erl_port_task.c b/erts/emulator/beam/erl_port_task.c
index 3dc7c14faf..11027dda7a 100644
--- a/erts/emulator/beam/erl_port_task.c
+++ b/erts/emulator/beam/erl_port_task.c
@@ -576,8 +576,20 @@ erts_port_task_schedule(Eterm id,
     if (!pp->sched.taskq && !pp->sched.exe_taskq) {
 #ifdef ERTS_SMP
 	ErtsRunQueue *xrunq = erts_check_emigration_need(runq, ERTS_PORT_PRIO_LEVEL);
-	if (xrunq) {
-	    /* Port emigrated ... */
+	ERTS_SMP_LC_ASSERT(runq != xrunq);
+	if (runq != (ErtsRunQueue *) erts_smp_atomic_read_nob(&pp->run_queue)) {
+	    /*
+	     * Someone else migrated this port while we had the run-queue
+	     * lock on runq unlocked in erts_check_emigration_need(). Respect
+	     * that migration decision...
+	     */
+	    erts_smp_runq_unlock(runq);
+	    if (xrunq)
+		erts_smp_runq_unlock(xrunq);
+	    runq = erts_port_runq(pp);
+	}
+	else if (xrunq) {
+	    /* Emigrate port... */
 	    erts_smp_atomic_set_nob(&pp->run_queue, (erts_aint_t) xrunq);
 	    erts_smp_runq_unlock(runq);
 	    runq = xrunq;
@@ -929,6 +941,8 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 
 #ifdef ERTS_SMP
 	xrunq = erts_check_emigration_need(runq, ERTS_PORT_PRIO_LEVEL);
+	ERTS_SMP_LC_ASSERT(runq != xrunq);
+	ERTS_SMP_LC_ASSERT(runq == (ErtsRunQueue *) erts_smp_atomic_read_nob(&pp->run_queue));
 	if (!xrunq) {
 #endif
 	    enqueue_port(runq, pp);
@@ -938,7 +952,7 @@ erts_port_task_execute(ErtsRunQueue *runq, Port **curr_port_pp)
 #ifdef ERTS_SMP
 	}
 	else {
-	    /* Port emigrated ... */
+	    /* Emigrate port... */
 	    erts_smp_atomic_set_nob(&pp->run_queue, (erts_aint_t) xrunq);
 	    enqueue_port(xrunq, pp);
 	    ASSERT(pp->sched.exe_taskq);