tree cf0e2cbeb389e3ef8d63b00bd76dc5cf0aa1c93b
parent 70bd9801911f8ed27df410d36a928166c37a68fd
author Ian Lance Taylor <iant@golang.org> 1536530002 -0700
committer Ian Lance Taylor <iant@golang.org> 1536859763 +0000

runtime: avoid write barriers with traceback info

Unlike the gc runtime, libgo stores traceback information in location
structs, which contain strings.  Therefore, copying location structs
around appears to require write barriers, although in fact write
barriers are never important because the strings are never allocated
in Go memory.  They come from libbacktrace.

Some of the generated write barriers come at times when write barriers
are not permitted.  For example, exitsyscall, marked
nowritebarrierrec, calls exitsyscallfast which calls traceGoSysExit
which calls traceEvent which calls traceStackID which calls
trace.stackTab.put which copies location values into memory allocated
by tab.newStack.  This write barrier can be invoked when there is no
p, causing a crash.

This change fixes the problem by ensuring that location values are
copied around in the tracing code with no write barriers.

This was found by fixing the compiler to fully implement
//go:nowritebarrierrec; CL to follow.

Change-Id: Ia672fadb37c3334fdcb2af77cec2ffbbe813678a
Reviewed-on: https://go-review.googlesource.com/134226
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>