runtime: handle sigprof in stackBarrier

Currently, if a profiling signal happens in the middle of
stackBarrier, gentraceback may see inconsistencies between stkbar and
the barriers on the stack and it will certainly get the wrong return
PC for stackBarrier. In most cases, the return PC won't be a PC at all
and this will immediately abort the traceback (which is considered
okay for a sigprof), but if it happens to be a valid PC this may sent
gentraceback down a rabbit hole.

Fix this by detecting when the gentraceback starts in stackBarrier and
simulating the completion of the barrier to get the correct initial
frame.

Change-Id: Ib11f705ac9194925f63fe5dfbfc84013a38333e6
Reviewed-on: https://go-review.googlesource.com/17035Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/traceback.go

@@ -190,6 +190,34 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 	}
 
 	f := findfunc(frame.pc)
+	if f.entry == stackBarrierPC {
+		// We got caught in the middle of a stack barrier
+		// (presumably by a signal), so stkbar may be
+		// inconsistent with the barriers on the stack.
+		// Simulate the completion of the barrier.
+		//
+		// On x86, SP will be exactly one word above
+		// savedLRPtr. On LR machines, SP will be above
+		// savedLRPtr by some frame size.
+		var stkbarPos uintptr
+		if len(stkbar) > 0 && stkbar[0].savedLRPtr < sp0 {
+			// stackBarrier has not incremented stkbarPos.
+			stkbarPos = gp.stkbarPos
+		} else if gp.stkbarPos > 0 && gp.stkbar[gp.stkbarPos-1].savedLRPtr < sp0 {
+			// stackBarrier has incremented stkbarPos.
+			stkbarPos = gp.stkbarPos - 1
+		} else {
+			printlock()
+			print("runtime: failed to unwind through stackBarrier at SP ", hex(sp0), " index ", gp.stkbarPos, "; ")
+			gcPrintStkbars(gp.stkbar)
+			print("\n")
+			throw("inconsistent state in stackBarrier")
+		}
+
+		frame.pc = gp.stkbar[stkbarPos].savedLRVal
+		stkbar = gp.stkbar[stkbarPos+1:]
+		f = findfunc(frame.pc)
+	}
 	if f == nil {
 		if callback != nil {
 			print("runtime: unknown pc ", hex(frame.pc), "\n")