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runtime: write barrier incorrect on wasm [1.11 backport] #30872

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gopherbot opened this issue Mar 15, 2019 · 3 comments

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commented Mar 15, 2019

@aclements requested issue #30871 to be considered for backport to the next 1.11 minor release.

@gopherbot, please backport to Go 1.12 and Go 1.11.

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commented Mar 15, 2019

Change https://golang.org/cl/167746 mentions this issue: [release-branch.go1.11] runtime: fix write barrier on wasm

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commented Mar 19, 2019

Approved, re: #30873 (comment)

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commented Mar 19, 2019

Closed by merging 987e905 to release-branch.go1.11.

@gopherbot gopherbot closed this Mar 19, 2019

gopherbot pushed a commit that referenced this issue Mar 19, 2019

[release-branch.go1.11] 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 #30872.

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 d9db9e3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/167746
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
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