runtime: enable write barriers during concurrent scan

Currently, write barriers are only enabled after completion of the
concurrent scan phase, as we enter the concurrent mark phase. However,
stack barriers are installed during the scan phase and assume that
write barriers will track changes to frames above the stack
barriers. Since write barriers aren't enabled until after stack
barriers are installed, we may miss modifications to the stack that
happen after installing the stack barriers and before enabling write
barriers.

Fix this by enabling write barriers during the scan phase.

This commit intentionally makes the minimal change to do this (there's
only one line of code change; the rest are comment changes). At the
very least, we should consider eliminating the ragged barrier that's
intended to synchronize the enabling of write barriers, but now just
wastes time. I've included a large comment about extensions and
alternative designs.

Change-Id: Ib20fede794e4fcb91ddf36f99bd97344d7f96421
Reviewed-on: https://go-review.googlesource.com/10795Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -64,10 +64,18 @@
 //     Once all the P's are aware of the new phase they will scan gs on preemption.
 //     This means that the scanning of preempted gs can't start until all the Ps
 //     have acknowledged.
+//     When a stack is scanned, this phase also installs stack barriers to
+//     track how much of the stack has been active.
+//     This transition enables write barriers because stack barriers
+//     assume that writes to higher frames will be tracked by write
+//     barriers. Technically this only needs write barriers for writes
+//     to stack slots, but we enable write barriers in general.
 // GCscan to GCmark
-//     GCMark turns on the write barrier which also only greys objects. No scanning
-//     of objects (making them black) can happen until all the Ps have acknowledged
-//     the phase change.
+//     In GCmark, work buffers are drained until there are no more
+//     pointers to scan.
+//     No scanning of objects (making them black) can happen until all
+//     Ps have enabled the write barrier, but that already happened in
+//     the transition to GCscan.
 // GCmark to GCmarktermination
 //     The only change here is that we start allocating black so the Ps must acknowledge
 //     the change before we begin the termination algorithm
@@ -220,7 +228,7 @@ var gcBlackenEnabled uint32
 const (
 	_GCoff             = iota // GC not running, write barrier disabled
 	_GCstw                    // unused state
-	_GCscan                   // GC collecting roots into workbufs, write barrier disabled
+	_GCscan                   // GC collecting roots into workbufs, write barrier ENABLED
 	_GCmark                   // GC marking from workbufs, write barrier ENABLED
 	_GCmarktermination        // GC mark termination: allocate black, P's help GC, write barrier ENABLED
 	_GCsweep                  // GC mark completed; sweeping in background, write barrier disabled
@@ -229,7 +237,7 @@ const (
 //go:nosplit
 func setGCPhase(x uint32) {
 	atomicstore(&gcphase, x)
-	writeBarrierEnabled = gcphase == _GCmark || gcphase == _GCmarktermination
+	writeBarrierEnabled = gcphase == _GCmark || gcphase == _GCmarktermination || gcphase == _GCscan
 }
 
 // gcMarkWorkerMode represents the mode that a concurrent mark worker
@@ -833,6 +841,31 @@ func gc(mode int) {
 		heapGoal = gcController.heapGoal
 
 		systemstack(func() {
+			// Enter scan phase. This enables write
+			// barriers to track changes to stack frames
+			// above the stack barrier.
+			//
+			// TODO: This has evolved to the point where
+			// we carefully ensure invariants we no longer
+			// depend on. Either:
+			//
+			// 1) Enable full write barriers for the scan,
+			// but eliminate the ragged barrier below
+			// (since the start the world ensures all Ps
+			// have observed the write barrier enable) and
+			// consider draining during the scan.
+			//
+			// 2) Only enable write barriers for writes to
+			// the stack at this point, and then enable
+			// write barriers for heap writes when we
+			// enter the mark phase. This means we cannot
+			// drain in the scan phase and must perform a
+			// ragged barrier to ensure all Ps have
+			// enabled heap write barriers before we drain
+			// or enable assists.
+			//
+			// 3) Don't install stack barriers over frame
+			// boundaries where there are up-pointers.
 			setGCPhase(_GCscan)
 
 			// Concurrent scan.
@@ -842,8 +875,7 @@ func gc(mode int) {
 			}
 			gcscan_m()
 
-			// Enter mark phase. This enables write
-			// barriers.
+			// Enter mark phase.
 			if debug.gctrace > 0 {
 				tInstallWB = nanotime()
 			}