1671572 - ThreadSanitizer: data race [@ GetCompositorBackendType] vs. [@ memcpy]

Status: RESOLVED WORKSFORME

(Core :: Graphics: Layers, defect)

Description

[Christian Holler (:decoder)]

The attached crash information was detected while running CI tests with ThreadSanitizer on try revision 1fdd4f52d08fd1fb01b6c77c4a86b03af275855d.

This came up during try runs for mochitest-media and looks bad to me: We seem to be reading mTextureFactoryIdentifier while at the same time assigning it (non-atomically, with memcpy(!)). I assume this could cause crashes or other bugs when reading a half set/updated mTextureFactoryIdentifier.

General information about TSan reports

Why fix races?

Data races are undefined behavior and can cause crashes as well as correctness issues. Compiler optimizations can cause racy code to have unpredictable and hard-to-reproduce behavior.

Rating

If you think this race can cause crashes or correctness issues, it would be great to rate the bug appropriately as P1/P2 and/or indicating this in the bug. This makes it a lot easier for us to assess the actual impact that these reports make and if they are helpful to you.

False Positives / Benign Races

Typically, races reported by TSan are not false positives [1], but it is possible that the race is benign. Even in this case it would be nice to come up with a fix if it is easily doable and does not regress performance. Every race that we cannot fix will have to remain on the suppression list and slows down the overall TSan performance. Also note that seemingly benign races can possibly be harmful (also depending on the compiler, optimizations and the architecture) [2][3].

[1] One major exception is the involvement of uninstrumented code from third-party libraries.
[2] http://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
[3] How to miscompile programs with "benign" data races: https://www.usenix.org/legacy/events/hotpar11/tech/final_files/Boehm.pdf

Suppressing unfixable races

If the bug cannot be fixed, then a runtime suppression needs to be added in mozglue/build/TsanOptions.cpp. The suppressions match on the full stack, so it should be picked such that it is unique to this particular race. The bug number of this bug should also be included so we have some documentation on why this suppression was added.

Comment 1

[Christian Holler (:decoder)]

Attachment: Detailed Crash Information

Comment 2

[Byron Campen [:bwc]]

Just saw this myself, probably (the memcpy side of the race didn't have a stack):

https://treeherder.mozilla.org/#/jobs?repo=try&revision=30dc65bffef4ee5627374df6214de824d9deaa61&selectedTaskRun=Y2naKum-Sj2osOeENvps5g.0

Comment 3

[Christian Holler (:decoder)]

Attachment: Bug 1671572 - Add temporary suppression. r?gankro

Depends on D94285

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/5045ae3d1a68
Add temporary suppression. r=Gankro

Comment 5

[Dorel Luca [:dluca]]

https://hg.mozilla.org/mozilla-central/rev/5045ae3d1a68

Comment 6

[Kris Wright :KrisWright]

Attachment: Bug 1671572 - Remove suppression for fixed issue.

It looks like this may have been fixed around 2 months ago in Bug 1664831: https://searchfox.org/mozilla-central/rev/1ebc9745be02eebf7a694f5c527a44a045b8c97a/gfx/layers/ipc/ShadowLayers.cpp#149

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/029d21a00d37
Remove suppression for fixed issue. r=decoder

Comment 8

[Sandor Molnar[:smolnar]]

https://hg.mozilla.org/mozilla-central/rev/029d21a00d37