1606807 - ThreadSanitizer: data race [@ mozilla::layers::AsyncPanZoomController::ReportCheckerboard] vs. [@ SetState]

Status: RESOLVED FIXED

(Core :: Panning and Zooming, defect, P3)

Description

[Christian Holler (:decoder)]

The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision d1ac49b9eb3e.

Looks like a race on mState being read and updated at the same time, maybe another missing lock?

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

[Botond Ballo [:botond]]

Attachment: Bug 1606807 - Protect the access of AsyncPanZoomController::mState in ReportCheckerboard(). r=tnikkel

Comment 3

[Botond Ballo [:botond]]

Oops, that fix doesn't quite work. It appears we have a "checkerboard event lock" (which the lock ordering documentation doesn't mention at all), and acquiring the APZC lock while holding the checkerboard event lock leads to a deadlock.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/757b5c587bdd
Protect the access of AsyncPanZoomController::mState in ReportCheckerboard(). r=tnikkel

Comment 5

[Arthur Iakab [arthur_iakab]]

https://hg.mozilla.org/mozilla-central/rev/757b5c587bdd