runtime: add new deltimer function

Updates #27707

Change-Id: I720e8af9e183c75abcb63ccc30466734c8dba74f
Reviewed-on: https://go-review.googlesource.com/c/go/+/171831
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

diff --git a/src/runtime/time.go b/src/runtime/time.go
index b4f1c0205e79d5f1ac1c3961ce1d0064453fb656..f2dd40e6b4ae3a698aeced66555fc59dc36c69ca 100644 (file)
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -8,6 +8,7 @@ package runtime
 
 import (
        "internal/cpu"
+       "runtime/internal/atomic"
        "unsafe"
 )
 
@@ -120,6 +121,16 @@ type timersBucket struct {
 // addtimer:
 //   timerNoStatus   -> timerWaiting
 //   anything else   -> panic: invalid value
+// deltimer:
+//   timerWaiting    -> timerDeleted
+//   timerModifiedXX -> timerDeleted
+//   timerNoStatus   -> do nothing
+//   timerDeleted    -> do nothing
+//   timerRemoving   -> do nothing
+//   timerRemoved    -> do nothing
+//   timerRunning    -> wait until status changes
+//   timerMoving     -> wait until status changes
+//   timerModifying  -> panic: concurrent deltimer/modtimer calls
 
 // Values for the timer status field.
 const (
@@ -216,8 +227,8 @@ func startTimer(t *timer) {
        addtimer(t)
 }
 
-// stopTimer removes t from the timer heap if it is there.
-// It returns true if t was removed, false if t wasn't even there.
+// stopTimer stops a timer.
+// It reports whether t was stopped before being run.
 //go:linkname stopTimer time.stopTimer
 func stopTimer(t *timer) bool {
        return deltimer(t)
@@ -335,14 +346,48 @@ func (tb *timersBucket) addtimerLocked(t *timer) bool {
        return true
 }
 
-// Delete timer t from the heap.
-// Do not need to update the timerproc: if it wakes up early, no big deal.
+// deltimer deletes the timer t. It may be on some other P, so we can't
+// actually remove it from the timers heap. We can only mark it as deleted.
+// It will be removed in due course by the P whose heap it is on.
+// Reports whether the timer was removed before it was run.
 func deltimer(t *timer) bool {
        if oldTimers {
                return deltimerOld(t)
        }
-       throw("no deltimer not yet implemented")
-       return false
+
+       for {
+               switch s := atomic.Load(&t.status); s {
+               case timerWaiting, timerModifiedLater:
+                       if atomic.Cas(&t.status, s, timerDeleted) {
+                               // Timer was not yet run.
+                               return true
+                       }
+               case timerModifiedEarlier:
+                       if atomic.Cas(&t.status, s, timerModifying) {
+                               if !atomic.Cas(&t.status, timerModifying, timerDeleted) {
+                                       badTimer()
+                               }
+                               // Timer was not yet run.
+                               return true
+                       }
+               case timerDeleted, timerRemoving, timerRemoved:
+                       // Timer was already run.
+                       return false
+               case timerRunning, timerMoving:
+                       // The timer is being run or moved, 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
+               case timerModifying:
+                       // Simultaneous calls to deltimer and modtimer.
+                       badTimer()
+               default:
+                       badTimer()
+               }
+       }
 }
 
 func deltimerOld(t *timer) bool {