rts: Close livelock window due to rapid ticker enable/disable

This fixes #11830, where the RTS would livelock if run with `-I0` due
to a regression introduced by bbdc52f3a6e6a28e209fb8f65699121d4ef3a4e3.
The reason for this is that the new codepath introduced a subtle race
condition:

 1. one thread could request that the ticker stop and would block until
    the ticker in fact stopped
 2. meanwhile, another thread could sneak in and restart the ticker

this was implemented in such a way where thread (1) would end up
blocked forever. The solution here is to simply not block. The worst
that will happen is that timer fires again, but is ignored since the
ticker is stopped.

Test Plan:
Validate, try reproduction case in #11830. Need to find a nice
testcase.

Reviewers: simonmar, erikd, hsyl20, austin

Reviewed By: erikd, hsyl20

Subscribers: erikd, thomie

Differential Revision: https://phabricator.haskell.org/D2129

GHC Trac Issues: #11830

1 files changed, 10 insertions, 6 deletions

diff --git a/rts/posix/Itimer.c b/rts/posix/Itimer.c
index 8915446814..770ff202ea 100644
--- a/rts/posix/Itimer.c
+++ b/rts/posix/Itimer.c
@@ -177,6 +177,7 @@ static void install_vtalrm_handler(TickProc handle_tick)
 #if defined(USE_PTHREAD_FOR_ITIMER)
 enum ItimerState {STOPPED, RUNNING, STOPPING, EXITED};
 static volatile enum ItimerState itimer_state = STOPPED;
+
 static void *itimer_thread_func(void *_handle_tick)
 {
     TickProc handle_tick = _handle_tick;
@@ -189,7 +190,7 @@ static void *itimer_thread_func(void *_handle_tick)
     it.it_value.tv_nsec = TimeToNS(itimer_interval) % 1000000000;
     it.it_interval = it.it_value;
 
-    timerfd = timerfd_create(CLOCK_MONOTONIC,TFD_CLOEXEC);
+    timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_CLOEXEC);
     if (timerfd == -1) {
         sysErrorBelch("timerfd_create");
         stg_exit(EXIT_FAILURE);
@@ -197,7 +198,7 @@ static void *itimer_thread_func(void *_handle_tick)
     if (!TFD_CLOEXEC) {
       fcntl(timerfd, F_SETFD, FD_CLOEXEC);
     }
-    timerfd_settime(timerfd,0,&it,NULL);
+    int ret = timerfd_settime(timerfd, 0, &it, NULL);
 #endif
 
     while (1) {
@@ -270,6 +271,11 @@ void
 startTicker(void)
 {
 #if defined(USE_PTHREAD_FOR_ITIMER)
+    // sanity check
+    if (itimer_state == EXITED) {
+        sysErrorBelch("ITimer: Tried to start a dead timer!\n");
+        stg_exit(EXIT_FAILURE);
+    }
     itimer_state = RUNNING;
 #elif defined(USE_TIMER_CREATE)
     {
@@ -306,10 +312,8 @@ stopTicker(void)
 #if defined(USE_PTHREAD_FOR_ITIMER)
     if (itimer_state == RUNNING) {
         itimer_state = STOPPING;
-        /* Wait for the thread to confirm it won't generate another tick. */
-        write_barrier();
-        while (itimer_state != STOPPED)
-            sched_yield();
+        // Note that the timer may fire once more, but that's okay;
+        // handle_tick is only called when itimer_state == RUNNING
     }
 #elif defined(USE_TIMER_CREATE)
     struct itimerspec it;