1606860 - ThreadSanitizer: data race [@ majorGCCount] vs. [@ operator++]

Status: RESOLVED FIXED

(Core :: JavaScript: GC, defect, P2)

Description

[Christian Holler (:decoder)]

The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision c0a6eb95b65c.

This is a simple race on majorGCNumber being updated by a JS worker. Are these counters supposed to be thread-safe? Probably harmless but seeing it quite a few times in test runs.

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

[Jon Coppeard (:jonco)]

This is a data race. SourceCompressionTask::shouldStart() calls GCRuntime::majorGCCount() from another runtime's main thread.

Comment 3

[Jason Orendorff [:jorendorff]]

Jon, please set a priority.

Comment 4

[Jon Coppeard (:jonco)]

Attachment: Bug 1606860 - Tidy source compression logic behind an API r?sfink

This moves a bunch of stuff that uses the internals of the helper thread system
into HelperThreads.cpp and puts it behind an API.

Comment 5

[Jon Coppeard (:jonco)]

Attachment: Bug 1606860 - Only start handling source compression tasks from this runtime at the start of GC r?sfink

Depends on D90524

Comment 6

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/f1d652a722c4
Tidy source compression logic behind an API r=sfink
https://hg.mozilla.org/integration/autoland/rev/f94ab6567f6b
Only start handling source compression tasks from this runtime at the start of GC r=sfink

Comment 7

[Cristian Brindusan [:cbrindusan]]

https://hg.mozilla.org/mozilla-central/rev/f1d652a722c4
https://hg.mozilla.org/mozilla-central/rev/f94ab6567f6b

Comment 8

[Aria Beingessner [:Gankra]]

Attachment: Bug 1606860 - Remove supression for fixed issue. r?jonco

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/5fb057109e86
Remove supression for fixed issue. r=decoder

Comment 10

[Atila Butkovits]

https://hg.mozilla.org/mozilla-central/rev/5fb057109e86