commit 001a75a74c4a27901b0b536efe1be581612c52a9

Author: Shawn Walker-Salas <[email protected]>

Date:   Tue Aug 18 14:13:38 2015 -0700

    runtime/trace: fix tracing of blocking system calls

    The placement and invocation of traceGoSysCall when using

    entersyscallblock() instead of entersyscall() differs enough that the

    TestTraceSymbolize test can fail on some platforms.

    This change moves the invocation of traceGoSysCall for entersyscall() so

    that the same number of "frames to skip" are present in the trace as when

    entersyscallblock() is used ensuring system call traces remain identical

    regardless of internal implementation choices.

    Fixes golang/go#12056

    Change-Id: I8361e91aa3708f5053f98263dfe9feb8c5d1d969

    Reviewed-on: https://go-review.googlesource.com/13861

    Run-TryBot: Dmitry Vyukov <[email protected]>

    Reviewed-by: Dmitry Vyukov <[email protected]>

diff --git a/src/runtime/proc1.go b/src/runtime/proc1.go

index 35d9e86..4ffe11b 100644

--- a/src/runtime/proc1.go

+++ b/src/runtime/proc1.go

@@ -1823,10 +1823,6 @@ func reentersyscall(pc, sp uintptr) {

 	// but can have inconsistent g->sched, do not let GC observe it.

 	_g_.m.locks++

-	if trace.enabled {

-		systemstack(traceGoSysCall)

-	}

-

 	// Entersyscall must not call any function that might split/grow the stack.

 	// (See details in comment above.)

 	// Catch calls that might, by replacing the stack guard with something that

@@ -1846,6 +1842,14 @@ func reentersyscall(pc, sp uintptr) {

 		})

 	}

+	if trace.enabled {

+		systemstack(traceGoSysCall)

+		// systemstack itself clobbers g.sched.{pc,sp} and we might

+		// need them later when the G is genuinely blocked in a

+		// syscall

+		save(pc, sp)

+	}

+

 	if atomicload(&sched.sysmonwait) != 0 { // TODO: fast atomic

 		systemstack(entersyscall_sysmon)

 		save(pc, sp)

diff --git a/src/runtime/trace.go b/src/runtime/trace.go

index 29600b5..6631bc2 100644

--- a/src/runtime/trace.go

+++ b/src/runtime/trace.go

@@ -828,7 +828,7 @@ func traceGoUnpark(gp *g, skip int) {

 }

 func traceGoSysCall() {

-	traceEvent(traceEvGoSysCall, 4)

+	traceEvent(traceEvGoSysCall, 1)

 }

 func traceGoSysExit(seq uint64, ts int64) {