commit 92e78f7e8d1bc92cb91d18e912ee20076a4747aa
author Austin Clements <austin@google.com> Fri Mar 15 13:29:40 2019 -0400
committer Brad Fitzpatrick <bradfitz@golang.org> Fri Apr 05 18:47:42 2019 +0000
tree bc6e299753df710f29fc5d97fe54bc406607d29f
parent c4e9966a32beb2d1d5fca4cbbe8c7e6cd484f976

[release-branch.go1.12] runtime: fix write barrier on wasm

The current wasm write barrier implementation incorrectly implements
the "deletion" part of the barrier. It correctly greys the new value
of the pointer, but rather than also greying the old value of the
pointer, it greys the object containing the slot (which, since the old
value was just overwritten, is not going to contain the old value).

This can lead to unmarked, reachable objects.

Often, this is masked by other marking activity, but one specific
sequence that can lead to an unmarked object because of this bug is:

1. Initially, GC is off, object A is reachable from just one pointer
in the heap.

2. GC starts and scans the stack of goroutine G.

3. G copies the pointer to A on to its stack and overwrites the
pointer to A in the heap. (Now A is reachable only from G's stack.)

4. GC finishes while A is still reachable from G's stack.

With a functioning deletion barrier, step 3 causes A to be greyed.
Without a functioning deletion barrier, nothing causes A to be greyed,
so A will be freed even though it's still reachable from G's stack.

This CL fixes the wasm write barrier.

Fixes #30873.

Change-Id: I8a74ee517facd3aa9ad606e5424bcf8f0d78e754
Reviewed-on: https://go-review.googlesource.com/c/go/+/167743
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit d9db9e32e924a60bbfbb15cc0dd7cfaaf8a62a3b)
Reviewed-on: https://go-review.googlesource.com/c/go/+/167745
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>