RTS: try to fix timer races

* Pthread based timer was initialized started while some other parts of
  the RTS assume it is initialized stopped, e.g. in hs_init_ghc:

     /* Start the "ticker" and profiling timer but don't start until the
     * scheduler is up. However, the ticker itself needs to be initialized
     * before the scheduler to ensure that the ticker mutex is initialized as
     * moreCapabilities will attempt to acquire it.
     */

* after a fork, don't start the timer before the IOManager is
  initialized: the timer handler (handle_tick) might call wakeUpRts to
  perform an idle GC, which calls wakeupIOManager/ioManagerWakeup

Found while debugging #18033/#20132 but I couldn't confirm if it fixes
them.

2 files changed, 5 insertions, 2 deletions

diff --git a/rts/Schedule.c b/rts/Schedule.c
index e0631482c9..c5a5041587 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -2194,7 +2194,6 @@ forkProcess(HsStablePtr *entry
         // On Unix, all timers are reset in the child, so we need to start
         // the timer again.
         initTimer();
-        startTimer();
 
         // TODO: need to trace various other things in the child
         // like startup event, capabilities, process info etc
@@ -2202,6 +2201,10 @@ forkProcess(HsStablePtr *entry
 
         initIOManagerAfterFork(&cap);
 
+        // start timer after the IOManager is initialized
+        // (the idle GC may wake up the IOManager)
+        startTimer();
+
         // Install toplevel exception handlers, so interruption
         // signal will be sent to the main thread.
         // See #12903
diff --git a/rts/posix/itimer/Pthread.c b/rts/posix/itimer/Pthread.c
index 5098d4f117..84501acb6f 100644
--- a/rts/posix/itimer/Pthread.c
+++ b/rts/posix/itimer/Pthread.c
@@ -169,7 +169,7 @@ void
 initTicker (Time interval, TickProc handle_tick)
 {
     itimer_interval = interval;
-    stopped = false;
+    stopped = true;
     exited = false;
 #if defined(HAVE_SIGNAL_H)
     sigset_t mask, omask;