rts: Pause timer while changing capability count

This avoids #17289.

2 files changed, 21 insertions, 11 deletions

diff --git a/rts/Schedule.c b/rts/Schedule.c
index 41d0dba953..3f04eb2af1 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -2236,6 +2236,12 @@ setNumCapabilities (uint32_t new_n_capabilities USED_IF_THREADS)
     cap = rts_lock();
     task = cap->running_task;
 
+
+    // N.B. We must stop the interval timer while we are changing the
+    // capabilities array lest handle_tick may try to context switch
+    // an old capability. See #17289.
+    stopTimer();
+
     stopAllCapabilities(&cap, task);
 
     if (new_n_capabilities < enabled_capabilities)
@@ -2318,6 +2324,8 @@ setNumCapabilities (uint32_t new_n_capabilities USED_IF_THREADS)
     // Notify IO manager that the number of capabilities has changed.
     rts_evalIO(&cap, ioManagerCapabilitiesChanged_closure, NULL);
 
+    startTimer();
+
     rts_unlock(cap);
 
 #endif // THREADED_RTS
diff --git a/rts/Timer.c b/rts/Timer.c
index 057cffc7c9..63854d7dda 100644
--- a/rts/Timer.c
+++ b/rts/Timer.c
@@ -25,6 +25,15 @@
 #include "Capability.h"
 #include "RtsSignals.h"
 
+// This global counter is used to allow multiple threads to stop the
+// timer temporarily with a stopTimer()/startTimer() pair.  If
+//      timer_enabled  == 0          timer is enabled
+//      timer_disabled == N, N > 0   timer is disabled by N threads
+// When timer_enabled makes a transition to 0, we enable the timer,
+// and when it makes a transition to non-0 we disable it.
+
+static StgWord timer_disabled;
+
 /* ticks left before next pre-emptive context switch */
 static int ticks_to_ctxt_switch = 0;
 
@@ -92,7 +101,9 @@ void
 handle_tick(int unused STG_UNUSED)
 {
   handleProfTick();
-  if (RtsFlags.ConcFlags.ctxtSwitchTicks > 0) {
+  if (RtsFlags.ConcFlags.ctxtSwitchTicks > 0
+      && RELAXED_LOAD(&timer_disabled) == 0)
+  {
       ticks_to_ctxt_switch--;
       if (ticks_to_ctxt_switch <= 0) {
           ticks_to_ctxt_switch = RtsFlags.ConcFlags.ctxtSwitchTicks;
@@ -148,15 +159,6 @@ handle_tick(int unused STG_UNUSED)
   }
 }
 
-// This global counter is used to allow multiple threads to stop the
-// timer temporarily with a stopTimer()/startTimer() pair.  If
-//      timer_enabled  == 0          timer is enabled
-//      timer_disabled == N, N > 0   timer is disabled by N threads
-// When timer_enabled makes a transition to 0, we enable the timer,
-// and when it makes a transition to non-0 we disable it.
-
-static StgWord timer_disabled;
-
 void
 initTimer(void)
 {
@@ -164,7 +166,7 @@ initTimer(void)
     if (RtsFlags.MiscFlags.tickInterval != 0) {
         initTicker(RtsFlags.MiscFlags.tickInterval, handle_tick);
     }
-    timer_disabled = 1;
+    RELAXED_STORE(&timer_disabled, 1);
 }
 
 void