Revert "runtime: don't hold worldsema across mark phase"

This reverts commit 7b294cdd8df0a9523010f6ffc80c59e64578f34b, CL 182657.

Reason for revert: This change may be causing latency problems
for applications which call ReadMemStats, because it may cause
all goroutines to stop until the GC completes.

https://golang.org/cl/215157 fixes this problem, but it's too
late in the cycle to land that.

Updates #19812.

Change-Id: Iaa26f4dec9b06b9db2a771a44e45f58d0aa8f26d
Reviewed-on: https://go-review.googlesource.com/c/go/+/216358
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 7 insertions, 10 deletions

diff --git a/src/runtime/trace.go b/src/runtime/trace.go
index 9aa9facabe..67a84425a8 100644
--- a/src/runtime/trace.go
+++ b/src/runtime/trace.go
@@ -180,12 +180,9 @@ func traceBufPtrOf(b *traceBuf) traceBufPtr {
 // Most clients should use the runtime/trace package or the testing package's
 // -test.trace flag instead of calling StartTrace directly.
 func StartTrace() error {
-	// Stop the world so that we can take a consistent snapshot
+	// Stop the world, so that we can take a consistent snapshot
 	// of all goroutines at the beginning of the trace.
-	// Do not stop the world during GC so we ensure we always see
-	// a consistent view of GC-related events (e.g. a start is always
-	// paired with an end).
-	stopTheWorldGC("start tracing")
+	stopTheWorld("start tracing")
 
 	// We are in stop-the-world, but syscalls can finish and write to trace concurrently.
 	// Exitsyscall could check trace.enabled long before and then suddenly wake up
@@ -196,7 +193,7 @@ func StartTrace() error {
 
 	if trace.enabled || trace.shutdown {
 		unlock(&trace.bufLock)
-		startTheWorldGC()
+		startTheWorld()
 		return errorString("tracing is already enabled")
 	}
 
@@ -267,7 +264,7 @@ func StartTrace() error {
 
 	unlock(&trace.bufLock)
 
-	startTheWorldGC()
+	startTheWorld()
 	return nil
 }
 
@@ -276,14 +273,14 @@ func StartTrace() error {
 func StopTrace() {
 	// Stop the world so that we can collect the trace buffers from all p's below,
 	// and also to avoid races with traceEvent.
-	stopTheWorldGC("stop tracing")
+	stopTheWorld("stop tracing")
 
 	// See the comment in StartTrace.
 	lock(&trace.bufLock)
 
 	if !trace.enabled {
 		unlock(&trace.bufLock)
-		startTheWorldGC()
+		startTheWorld()
 		return
 	}
 
@@ -320,7 +317,7 @@ func StopTrace() {
 	trace.shutdown = true
 	unlock(&trace.bufLock)
 
-	startTheWorldGC()
+	startTheWorld()
 
 	// The world is started but we've set trace.shutdown, so new tracing can't start.
 	// Wait for the trace reader to flush pending buffers and stop.