runtime: replace scanblock(0, 0, nil, nil) with drainworkbuf

scanblock(0, 0, nil, nil) was just a confusing way of saying

  wbuf = getpartialorempty()
  drainworkbuf(wbuf, true)

Make drainworkbuf accept a nil workbuf and perform the
getpartialorempty itself and replace all uses of scanblock(0, 0, nil,
nil) with direct calls to drainworkbuf(nil, true).

Change-Id: I7002a2f8f3eaf6aa85bbf17ccc81d7288acfef1c
Reviewed-on: https://go-review.googlesource.com/4781
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index afca568e0bc9216fb47e4ad9ec5b9985cd44d360..3f361c1b4d9cf8340fb9ec16228982ae0390e0a8 100644 (file)
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -461,6 +461,9 @@ func scanblock(b0, n0 uintptr, ptrmask *uint8, wbuf *workbuf) *workbuf {
 // If drainallwbufs is true find all other available workbufs and repeat the process.
 //go:nowritebarrier
 func drainworkbuf(wbuf *workbuf, drainallwbufs bool) {
+       if wbuf == nil {
+               wbuf = getpartialorempty(472)
+       }
        checknocurrentwbuf()
        if gcphase != _GCmark && gcphase != _GCmarktermination {
                throw("scanblock phase incorrect")
@@ -1149,10 +1152,7 @@ func gchelper() {
        // parallel mark for over GC roots
        parfordo(work.markfor)
        if gcphase != _GCscan {
-               wbuf := scanblock(0, 0, nil, nil) // blocks in getfull
-               if wbuf != nil {
-                       throw("gchelper did not extinguish wbuf")
-               }
+               drainworkbuf(nil, true) // blocks in getfull
        }
 
        if trace.enabled {
@@ -1414,13 +1414,9 @@ func gcscan_m() {
 // This is the concurrent mark phase.
 //go:nowritebarrier
 func gcmark_m() {
-       // If one is available grab this M's workbuffer.
-       wbuf := scanblock(0, 0, nil, nil)
-       if wbuf != nil {
-               throw("gcmark_m did not extinguish wbuf")
-       }
+       drainworkbuf(nil, true)
        // TODO add another harvestwbuf and reset work.nwait=0, work.ndone=0, and work.nproc=1
-       // and repeat the above scanblock.
+       // and repeat the above drainworkbuf.
 }
 
 // For now this must be bracketed with a stoptheworld and a starttheworld to ensure
@@ -1505,10 +1501,7 @@ func gc(start_time int64, eagersweep bool) {
        harvestwbufs() // move local workbufs onto global queues where the GC can find them
        gchelperstart()
        parfordo(work.markfor)
-       wbuf := scanblock(0, 0, nil, nil)
-       if wbuf != nil {
-               throw("gc does not extinguish wbuf")
-       }
+       drainworkbuf(nil, true)
 
        if work.full != 0 {
                throw("work.full != 0")