runtime: use current P's race context in timer code

We were using the race context of the P that held the timer,
but since we unlock the P's timers while executing a timer
that could lead to a race on the race context itself.

Updates #6239
Updates #27707
Fixes #35906

Change-Id: I5f9d5f52d8e28dffb88c3327301071b16ed1a913
Reviewed-on: https://go-review.googlesource.com/c/go/+/209580
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
diff --git a/src/runtime/race/timer_test.go b/src/runtime/race/timer_test.go
new file mode 100644
index 0000000..a6c34a8
--- /dev/null
+++ b/src/runtime/race/timer_test.go
@@ -0,0 +1,33 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build race
+
+package race_test
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestTimers(t *testing.T) {
+	const goroutines = 8
+	var wg sync.WaitGroup
+	wg.Add(goroutines)
+	var mu sync.Mutex
+	for i := 0; i < goroutines; i++ {
+		go func() {
+			defer wg.Done()
+			ticker := time.NewTicker(1)
+			defer ticker.Stop()
+			for c := 0; c < 1000; c++ {
+				<-ticker.C
+				mu.Lock()
+				mu.Unlock()
+			}
+		}()
+	}
+	wg.Wait()
+}
diff --git a/src/runtime/time.go b/src/runtime/time.go
index a7d14cf..d64bea8 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -805,10 +805,11 @@
 //go:systemstack
 func runOneTimer(pp *p, t *timer, now int64) {
 	if raceenabled {
-		if pp.timerRaceCtx == 0 {
-			pp.timerRaceCtx = racegostart(funcPC(runtimer) + sys.PCQuantum)
+		ppcur := getg().m.p.ptr()
+		if ppcur.timerRaceCtx == 0 {
+			ppcur.timerRaceCtx = racegostart(funcPC(runtimer) + sys.PCQuantum)
 		}
-		raceacquirectx(pp.timerRaceCtx, unsafe.Pointer(t))
+		raceacquirectx(ppcur.timerRaceCtx, unsafe.Pointer(t))
 	}
 
 	f := t.f
@@ -836,12 +837,12 @@
 	}
 
 	if raceenabled {
-		// Temporarily use the P's racectx for g0.
+		// Temporarily use the current P's racectx for g0.
 		gp := getg()
 		if gp.racectx != 0 {
 			throw("runOneTimer: unexpected racectx")
 		}
-		gp.racectx = pp.timerRaceCtx
+		gp.racectx = gp.m.p.ptr().timerRaceCtx
 	}
 
 	unlock(&pp.timersLock)