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runtime: missing page mark in write barrier #39432

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aclements opened this issue Jun 6, 2020 · 7 comments
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runtime: missing page mark in write barrier #39432

aclements opened this issue Jun 6, 2020 · 7 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. release-blocker
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Go1.15

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@aclements
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aclements commented Jun 6, 2020 •
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There are two places in the runtime where we set mark bits: greyobject and wbBufFlush1. The page sweeper depends on page marks being set whenever any object in a span is marked, but we currently only set page marks in greyobject. This means if all of the reachable objects on a span are marked by the write barrier, the page mark for the span won't be set, and the page sweeper will incorrectly reclaim the span. This is fairly unlikely, but possible, especially if very few objects in a span are actually live.

I found this by chance when I was reorganizing these two functions. I'm fairly certainly this has been a bug since page marks were introduced in Go 1.12.

/cc @mknyszek
@aclements aclements added release-blocker NeedsFix The path to resolution is known, but the work has not been done. labels Jun 6, 2020
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Actually, we're also missing the page mark in gcmarknewobject.

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@gopherbot, please open backport issues for 1.13 and 1.14

@aclements aclements added this to the Go1.15 milestone Jun 6, 2020
@gopherbot gopherbot mentioned this issue Jun 6, 2020
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Backport issue(s) opened: #39433 (for 1.13), #39434 (for 1.14).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

@gopherbot gopherbot mentioned this issue Jun 6, 2020
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@aclements aclements self-assigned this Jun 6, 2020
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Change https://golang.org/cl/236817 mentions this issue: runtime: always mark span when marking an object

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Change https://golang.org/cl/236922 mentions this issue: [release-branch.go1.14] runtime: always mark span when marking an object

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Change https://golang.org/cl/236923 mentions this issue: [release-branch.go1.13] runtime: always mark span when marking an object

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Ah! I was wrong. This is a performance bug, but not in fact a correctness bug.

The page sweeper uses the span marks to determine where it thinks sweeping will return whole spans, but it just calls sweep, and sweep will do the appropriate thing for any span. Hence, the span marks are really just a hint (albeit an important one for the performance of the sweeper). Since we already mostly got the span marks right, I don't think the performance bug here is severe, and hence not worth the backports.

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