Currently, if sigprof determines that the G is in user code (not cgo
or libcall code), it will only traceback the G stack if it can acquire
the stack barrier lock. However, it has no such restriction if the G
is in cgo or libcall code. Because cgo calls count as syscalls, stack
scanning and stack barrier installation can occur during a cgo call,
which means sigprof could attempt to traceback a G in a cgo call while
scanstack is installing stack barriers in that G's stack. As a result,
the following sequence of events can cause the sigprof traceback to
panic with "missed stack barrier":

1. M1: G1 performs a Cgo call (which, on Windows, is any system call,
   which could explain why this is easier to reproduce on Windows).

2. M1: The Cgo call puts G1 into _Gsyscall state.

3. M2: GC starts a scan of G1's stack. It puts G1 in to _Gscansyscall
   and acquires the stack barrier lock.

4. M3: A profiling signal comes in. On Windows this is a global
   (though I don't think this matters), so the runtime stops M1 and
   calls sigprof for G1.

5. M3: sigprof fails to acquire the stack barrier lock (because the
   GC's stack scan holds it).

6. M3: sigprof observes that G1 is in a Cgo call, so it calls
   gentraceback on G1 with its Cgo transition point.

7. M3: gentraceback on G1 grabs the currently empty g.stkbar slice.

8. M2: GC finishes scanning G1's stack and installing stack barriers.

9. M3: gentraceback encounters one of the just-installed stack
   barriers and panics.

This commit fixes this by only allowing cgo tracebacks if sigprof can
acquire the stack barrier lock, just like in the regular user
traceback case.

For good measure, we put the same constraint on libcall tracebacks.
This case is probably already safe because, unlike cgo calls, libcalls
leave the G in _Grunning and prevent reaching a safe point, so
scanstack cannot run during a libcall. However, this also means that
sigprof will always acquire the stack barrier lock without contention,
so there's no cost to adding this constraint to libcall tracebacks.

Fixes #12528. For 1.5.3 (will require some backporting).

Change-Id: Ia5a4b8e3d66b23b02ffcd54c6315c81055c0cec2
Reviewed-on: https://go-review.googlesource.com/18023
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>