runtime: merge timerNoStatus into timerRemoved

For historical reasons, we have to treat a zero timer as
the same as an initialized timer that was stopped (removed).
The two states are already treated mostly identically.
Merge them.

This is part of a larger simplification of the state set.

[This is one CL in a refactoring stack making very small changes
in each step, so that any subtle bugs that we miss can be more
easily pinpointed to a small change.]

Change-Id: I9c3aeb8f92bafb18c47489c1ec20a7b87ac5cd9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/564122
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

diff --git a/src/runtime/time.go b/src/runtime/time.go
index 3ecfb90bfbc158327cbefa12f6fc683cfdb8c994..679a155beced8a02e68a0c0409f87610762d9096 100644 (file)
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -73,7 +73,6 @@ type timer struct {
 // deltimer:
 //   timerWaiting         -> timerModifying -> timerDeleted
 //   timerModified        -> timerModifying -> timerDeleted
-//   timerNoStatus        -> do nothing
 //   timerDeleted         -> do nothing
 //   timerRemoved         -> do nothing
 //   timerRunning         -> wait until status changes
@@ -81,7 +80,6 @@ type timer struct {
 // modtimer:
 //   timerWaiting    -> timerModifying -> timerModified
 //   timerModified   -> timerModifying -> timerModified
-//   timerNoStatus   -> timerModifying -> timerWaiting
 //   timerRemoved    -> timerModifying -> timerWaiting
 //   timerDeleted    -> timerModifying -> timerModified
 //   timerRunning    -> wait until status changes
@@ -90,9 +88,9 @@ type timer struct {
 //   timerDeleted    -> timerModifying -> timerRemoved
 //   timerModified   -> timerModifying -> timerWaiting
 // runtimer (looks in P's timer heap):
-//   timerNoStatus   -> panic: uninitialized timer
+//   timerRemoved   -> panic: uninitialized timer
 //   timerWaiting    -> timerWaiting or
-//   timerWaiting    -> timerRunning -> timerNoStatus or
+//   timerWaiting    -> timerRunning -> timerRemoved or
 //   timerWaiting    -> timerRunning -> timerWaiting
 //   timerModifying  -> wait until status changes
 //   timerModified   -> timerModifying -> timerWaiting
@@ -101,8 +99,9 @@ type timer struct {
 
 // Values for the timer status field.
 const (
-       // Timer has no status set yet.
-       timerNoStatus = iota
+       // Timer has no status set yet or is removed from the heap.
+       // Must be zero value; see issue 21874.
+       timerRemoved = iota
 
        // Waiting for timer to fire.
        // The timer is in some P's heap.
@@ -116,10 +115,6 @@ const (
        // It should not be run, but it is still in some P's heap.
        timerDeleted
 
-       // The timer has been stopped.
-       // It is not in any P's heap.
-       timerRemoved
-
        // The timer is being modified.
        // The timer will only have this status briefly.
        timerModifying
@@ -183,7 +178,7 @@ func startTimer(t *timer) {
        if raceenabled {
                racerelease(unsafe.Pointer(t))
        }
-       if t.status.Load() != timerNoStatus {
+       if t.status.Load() != 0 {
                throw("startTimer called with initialized timer")
        }
        resettimer(t, t.when)
@@ -278,10 +273,6 @@ func deltimer(t *timer) bool {
                        // The timer is being run or modified, by a different P.
                        // Wait for it to complete.
                        osyield()
-               case timerNoStatus:
-                       // Removing timer that was never added or
-                       // has already been run. Also see issue 21874.
-                       return false
                default:
                        badTimer()
                }
@@ -326,7 +317,7 @@ func modtimer(t *timer, when, period int64, f func(any, uintptr), arg any, seq u
                throw("timer period must be non-negative")
        }
 
-       status := uint32(timerNoStatus)
+       status := uint32(timerRemoved)
        wasRemoved := false
        var pending bool
        var mp *m
@@ -342,7 +333,7 @@ loop:
                                break loop
                        }
                        releasem(mp)
-               case timerNoStatus, timerRemoved:
+               case timerRemoved:
                        // Prevent preemption while the timer is in timerModifying.
                        // This could lead to a self-deadlock. See #38070.
                        mp = acquirem()
@@ -536,7 +527,7 @@ func moveTimers(pp *p, timers []*timer) {
                        case timerModifying:
                                // Loop until the modification is complete.
                                osyield()
-                       case timerNoStatus, timerRemoved:
+                       case timerRemoved:
                                // We should not see these status values in a timers heap.
                                badTimer()
                        case timerRunning:
@@ -604,7 +595,7 @@ func adjusttimers(pp *p, now int64, force bool) {
                                        badTimer()
                                }
                        }
-               case timerNoStatus, timerRunning, timerRemoved:
+               case timerRunning, timerRemoved:
                        badTimer()
                case timerWaiting:
                        // OK, nothing to do.
@@ -762,7 +753,7 @@ func runtimer(pp *p, now int64) int64 {
                        // Wait for modification to complete.
                        osyield()
 
-               case timerNoStatus, timerRemoved:
+               case timerRemoved:
                        // Should not see a new or inactive timer on the heap.
                        badTimer()
                case timerRunning:
@@ -808,7 +799,7 @@ func runOneTimer(pp *p, t *timer, now int64) {
        } else {
                // Remove from heap.
                dodeltimer0(pp)
-               if !t.status.CompareAndSwap(timerRunning, timerNoStatus) {
+               if !t.status.CompareAndSwap(timerRunning, timerRemoved) {
                        badTimer()
                }
        }