runtime: make next_gc atomically accessed

next_gc is mostly updated only during a STW, but may occasionally be
updated by calls to e.g. debug.SetGCPercent. In this case the update is
supposed to be protected by the heap lock, but in reality it's accessed
by gcController.revise which may be called without the heap lock held
(despite its documentation, which will be updated in a later change).

Change the synchronization policy on next_gc so that it's atomically
accessed when the world is not stopped to aid in making revise safe for
concurrent use.

Change-Id: I79657a72f91563f3241aaeda66e8a7757d399529
Reviewed-on: https://go-review.googlesource.com/c/go/+/246962
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mgc.go

@@ -409,7 +409,8 @@ type gcControllerState struct {
 }
 
 // startCycle resets the GC controller's state and computes estimates
-// for a new GC cycle. The caller must hold worldsema.
+// for a new GC cycle. The caller must hold worldsema and the world
+// must be stopped.
 func (c *gcControllerState) startCycle() {
 	c.scanWork = 0
 	c.bgScanCredit = 0
@@ -499,7 +500,7 @@ func (c *gcControllerState) revise() {
 
 	// Assume we're under the soft goal. Pace GC to complete at
 	// next_gc assuming the heap is in steady-state.
-	heapGoal := int64(memstats.next_gc)
+	heapGoal := int64(atomic.Load64(&memstats.next_gc))
 
 	// Compute the expected scan work remaining.
 	//
@@ -512,12 +513,12 @@ func (c *gcControllerState) revise() {
 	// 100*heap_scan.)
 	scanWorkExpected := int64(float64(scan) * 100 / float64(100+gcpercent))
 
-	if live > memstats.next_gc || work > scanWorkExpected {
+	if int64(live) > heapGoal || work > scanWorkExpected {
 		// We're past the soft goal, or we've already done more scan
 		// work than we expected. Pace GC so that in the worst case it
 		// will complete by the hard goal.
 		const maxOvershoot = 1.1
-		heapGoal = int64(float64(memstats.next_gc) * maxOvershoot)
+		heapGoal = int64(float64(heapGoal) * maxOvershoot)
 
 		// Compute the upper bound on the scan work remaining.
 		scanWorkExpected = int64(scan)
@@ -846,7 +847,7 @@ func gcSetTriggerRatio(triggerRatio float64) {
 
 	// Commit to the trigger and goal.
 	memstats.gc_trigger = trigger
-	memstats.next_gc = goal
+	atomic.Store64(&memstats.next_gc, goal)
 	if trace.enabled {
 		traceNextGC()
 	}
@@ -903,7 +904,7 @@ func gcSetTriggerRatio(triggerRatio float64) {
 //
 // mheap_.lock must be held or the world must be stopped.
 func gcEffectiveGrowthRatio() float64 {
-	egogc := float64(memstats.next_gc-memstats.heap_marked) / float64(memstats.heap_marked)
+	egogc := float64(atomic.Load64(&memstats.next_gc)-memstats.heap_marked) / float64(memstats.heap_marked)
 	if egogc < 0 {
 		// Shouldn't happen, but just in case.
 		egogc = 0

src/runtime/mgcscavenge.go

@@ -123,7 +123,7 @@ func gcPaceScavenger() {
 		return
 	}
 	// Compute our scavenging goal.
-	goalRatio := float64(memstats.next_gc) / float64(memstats.last_next_gc)
+	goalRatio := float64(atomic.Load64(&memstats.next_gc)) / float64(memstats.last_next_gc)
 	retainedGoal := uint64(float64(memstats.last_heap_inuse) * goalRatio)
 	// Add retainExtraPercent overhead to retainedGoal. This calculation
 	// looks strange but the purpose is to arrive at an integer division

src/runtime/mstats.go

@@ -57,9 +57,15 @@ type mstats struct {
 	gc_sys       uint64 // updated atomically or during STW
 	other_sys    uint64 // updated atomically or during STW
 
-	// Statistics about garbage collector.
+	// Statistics about the garbage collector.
+
+	// next_gc is the goal heap_live for when next GC ends.
+	// Set to ^uint64(0) if disabled.
+	//
+	// Read and written atomically, unless the world is stopped.
+	next_gc uint64
+
 	// Protected by mheap or stopping the world during GC.
-	next_gc         uint64 // goal heap_live for when next GC ends; ^0 if disabled
 	last_gc_unix    uint64 // last gc (in unix time)
 	pause_total_ns  uint64
 	pause_ns        [256]uint64 // circular buffer of recent gc pause lengths

src/runtime/trace.go

@@ -13,6 +13,7 @@
 package runtime
 
 import (
+	"runtime/internal/atomic"
 	"runtime/internal/sys"
 	"unsafe"
 )
@@ -1146,11 +1147,11 @@ func traceHeapAlloc() {
 }
 
 func traceNextGC() {
-	if memstats.next_gc == ^uint64(0) {
+	if nextGC := atomic.Load64(&memstats.next_gc); nextGC == ^uint64(0) {
 		// Heap-based triggering is disabled.
 		traceEvent(traceEvNextGC, -1, 0)
 	} else {
-		traceEvent(traceEvNextGC, -1, memstats.next_gc)
+		traceEvent(traceEvNextGC, -1, nextGC)
 	}
 }