runtime: acquire stack lock in traceEvent

traceEvent records system call events after a G has already entered
_Gsyscall, which means the garbage collector could be installing stack
barriers in the G's stack during the traceEvent. If traceEvent
attempts to capture the user stack during this, it may observe a
inconsistent stack barriers and panic. Fix this by acquiring the stack
lock around the stack walk in traceEvent.

Fixes #14101.

Change-Id: I15f0ab0c70c04c6e182221f65a6f761c5a896459
Reviewed-on: https://go-review.googlesource.com/18973
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/trace.go

@@ -529,7 +529,12 @@ func traceEvent(ev byte, skip int, args ...uint64) {
 			nstk = callers(skip, buf.stk[:])
 		} else if gp != nil {
 			gp = mp.curg
-			nstk = gcallers(gp, skip, buf.stk[:])
+			// This may happen when tracing a system call,
+			// so we must lock the stack.
+			if gcTryLockStackBarriers(gp) {
+				nstk = gcallers(gp, skip, buf.stk[:])
+				gcUnlockStackBarriers(gp)
+			}
 		}
 		if nstk > 0 {
 			nstk-- // skip runtime.goexit