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runtime: introduce a type for lfstacks

The lfstack API is still a C-style API: lfstacks all have unhelpful
type uint64 and the APIs are package-level functions. Make the code
more readable and Go-style by creating an lfstack type with methods
for push, pop, and empty.

Change-Id: I64685fa3be0e82ae2d1a782a452a50974440a827
Reviewed-on: https://go-review.googlesource.com/38290
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
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aclements committed Mar 7, 2017
1 parent 2805d20 commit 13ae271d5d007dcd630d9f43d6a43016b9af6e5c
Showing with 35 additions and 24 deletions.
  1. +2 −2 src/runtime/export_test.go
  2. +23 −12 src/runtime/lfstack.go
  3. +3 −3 src/runtime/mgc.go
  4. +7 −7 src/runtime/mgcwork.go
@@ -41,11 +41,11 @@ type LFNode struct {
}
func LFStackPush(head *uint64, node *LFNode) {
lfstackpush(head, (*lfnode)(unsafe.Pointer(node)))
(*lfstack)(head).push((*lfnode)(unsafe.Pointer(node)))
}
func LFStackPop(head *uint64) *LFNode {
return (*LFNode)(unsafe.Pointer(lfstackpop(head)))
return (*LFNode)(unsafe.Pointer((*lfstack)(head).pop()))
}
func GCMask(x interface{}) (ret []byte) {
@@ -3,10 +3,6 @@
// license that can be found in the LICENSE file.
// Lock-free stack.
// Initialize head to 0, compare with 0 to test for emptiness.
// The stack does not keep pointers to nodes,
// so they can be garbage collected if there are no other pointers to nodes.
// The following code runs only in non-preemptible contexts.
package runtime
@@ -15,32 +11,47 @@ import (
"unsafe"
)
func lfstackpush(head *uint64, node *lfnode) {
// lfstack is the head of a lock-free stack.
//
// The zero value of lfstack is an empty list.
//
// This stack is intrusive. Nodes must embed lfnode as the first field.
//
// The stack does not keep GC-visible pointers to nodes, so the caller
// is responsible for ensuring the nodes are not garbage collected
// (typically by allocating them from manually-managed memory).
type lfstack uint64
func (head *lfstack) push(node *lfnode) {
node.pushcnt++
new := lfstackPack(node, node.pushcnt)
if node1 := lfstackUnpack(new); node1 != node {
print("runtime: lfstackpush invalid packing: node=", node, " cnt=", hex(node.pushcnt), " packed=", hex(new), " -> node=", node1, "\n")
throw("lfstackpush")
print("runtime: lfstack.push invalid packing: node=", node, " cnt=", hex(node.pushcnt), " packed=", hex(new), " -> node=", node1, "\n")
throw("lfstack.push")
}
for {
old := atomic.Load64(head)
old := atomic.Load64((*uint64)(head))
node.next = old
if atomic.Cas64(head, old, new) {
if atomic.Cas64((*uint64)(head), old, new) {
break
}
}
}
func lfstackpop(head *uint64) unsafe.Pointer {
func (head *lfstack) pop() unsafe.Pointer {
for {
old := atomic.Load64(head)
old := atomic.Load64((*uint64)(head))
if old == 0 {
return nil
}
node := lfstackUnpack(old)
next := atomic.Load64(&node.next)
if atomic.Cas64(head, old, next) {
if atomic.Cas64((*uint64)(head), old, next) {
return unsafe.Pointer(node)
}
}
}
func (head *lfstack) empty() bool {
return atomic.Load64((*uint64)(head)) == 0
}
@@ -782,8 +782,8 @@ const gcAssistTimeSlack = 5000
const gcOverAssistWork = 64 << 10
var work struct {
full uint64 // lock-free list of full blocks workbuf
empty uint64 // lock-free list of empty blocks workbuf
full lfstack // lock-free list of full blocks workbuf
empty lfstack // lock-free list of empty blocks workbuf
pad0 [sys.CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait
// bytesMarked is the number of bytes marked this cycle. This
@@ -1574,7 +1574,7 @@ func gcMarkWorkAvailable(p *p) bool {
if p != nil && !p.gcw.empty() {
return true
}
if atomic.Load64(&work.full) != 0 {
if !work.full.empty() {
return true // global work available
}
if work.markrootNext < work.markrootJobs {
@@ -312,7 +312,7 @@ func (b *workbuf) checkempty() {
func getempty() *workbuf {
var b *workbuf
if work.empty != 0 {
b = (*workbuf)(lfstackpop(&work.empty))
b = (*workbuf)(work.empty.pop())
if b != nil {
b.checkempty()
}
@@ -324,11 +324,11 @@ func getempty() *workbuf {
}
// putempty puts a workbuf onto the work.empty list.
// Upon entry this go routine owns b. The lfstackpush relinquishes ownership.
// Upon entry this go routine owns b. The lfstack.push relinquishes ownership.
//go:nowritebarrier
func putempty(b *workbuf) {
b.checkempty()
lfstackpush(&work.empty, &b.node)
work.empty.push(&b.node)
}
// putfull puts the workbuf on the work.full list for the GC.
@@ -337,14 +337,14 @@ func putempty(b *workbuf) {
//go:nowritebarrier
func putfull(b *workbuf) {
b.checknonempty()
lfstackpush(&work.full, &b.node)
work.full.push(&b.node)
}
// trygetfull tries to get a full or partially empty workbuffer.
// If one is not immediately available return nil
//go:nowritebarrier
func trygetfull() *workbuf {
b := (*workbuf)(lfstackpop(&work.full))
b := (*workbuf)(work.full.pop())
if b != nil {
b.checknonempty()
return b
@@ -365,7 +365,7 @@ func trygetfull() *workbuf {
// phase.
//go:nowritebarrier
func getfull() *workbuf {
b := (*workbuf)(lfstackpop(&work.full))
b := (*workbuf)(work.full.pop())
if b != nil {
b.checknonempty()
return b
@@ -383,7 +383,7 @@ func getfull() *workbuf {
println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
}
b = (*workbuf)(lfstackpop(&work.full))
b = (*workbuf)(work.full.pop())
if b != nil {
b.checknonempty()
return b

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