runtime: replace *g with guintptr in trace

trace's reader *g is going to cause write barriers in unfortunate
places, so replace it with a guintptr.

Change-Id: Ie8fb13bb89a78238f9d2a77ec77da703e96df8af
Reviewed-on: https://go-review.googlesource.com/31469
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

diff --git a/src/runtime/trace.go b/src/runtime/trace.go
index 4c0f1de44bd231c2b34f509b1b56e35025bcefff..1eff814506eec0f6ce39c30578d94559b36faae0 100644 (file)
--- a/src/runtime/trace.go
+++ b/src/runtime/trace.go
@@ -112,7 +112,7 @@ var trace struct {
        empty         traceBufPtr // stack of empty buffers
        fullHead      traceBufPtr // queue of full buffers
        fullTail      traceBufPtr
-       reader        *g              // goroutine that called ReadTrace, or nil
+       reader        guintptr        // goroutine that called ReadTrace, or nil
        stackTab      traceStackTable // maps stack traces to unique ids
 
        // Dictionary for traceEvString.
@@ -313,7 +313,7 @@ func StopTrace() {
        if trace.fullHead != 0 || trace.fullTail != 0 {
                throw("trace: non-empty full trace buffer")
        }
-       if trace.reading != 0 || trace.reader != nil {
+       if trace.reading != 0 || trace.reader != 0 {
                throw("trace: reading after shutdown")
        }
        for trace.empty != 0 {
@@ -341,7 +341,7 @@ func ReadTrace() []byte {
        lock(&trace.lock)
        trace.lockOwner = getg()
 
-       if trace.reader != nil {
+       if trace.reader != 0 {
                // More than one goroutine reads trace. This is bad.
                // But we rather do not crash the program because of tracing,
                // because tracing can be enabled at runtime on prod servers.
@@ -365,7 +365,7 @@ func ReadTrace() []byte {
        }
        // Wait for new data.
        if trace.fullHead == 0 && !trace.shutdown {
-               trace.reader = getg()
+               trace.reader.set(getg())
                goparkunlock(&trace.lock, "trace reader (blocked)", traceEvGoBlock, 2)
                lock(&trace.lock)
        }
@@ -419,16 +419,16 @@ func ReadTrace() []byte {
 
 // traceReader returns the trace reader that should be woken up, if any.
 func traceReader() *g {
-       if trace.reader == nil || (trace.fullHead == 0 && !trace.shutdown) {
+       if trace.reader == 0 || (trace.fullHead == 0 && !trace.shutdown) {
                return nil
        }
        lock(&trace.lock)
-       if trace.reader == nil || (trace.fullHead == 0 && !trace.shutdown) {
+       if trace.reader == 0 || (trace.fullHead == 0 && !trace.shutdown) {
                unlock(&trace.lock)
                return nil
        }
-       gp := trace.reader
-       trace.reader = nil
+       gp := trace.reader.ptr()
+       trace.reader.set(nil)
        unlock(&trace.lock)
        return gp
 }