1647403 - ThreadSanitizer: data race [@ CrashStatsLogForwarder::UpdateStringsVector] vs. [@ CrashStatsLogForwarder::UpdateStringsVector]

Status: REOPENED

(Core :: Graphics, defect)

Description

[Jason Kratzer [:jkratzer]]

Attachment: log_ffp_asan_1404.log.3499.txt

The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision 20200619-d1a4f9157858.

For detailed crash information, see attachment.
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

[Aria Beingessner [:Gankra]]

Tyson, have y'all seen this at all since it was reported?

Comment 2

[Tyson Smith [:tsmith]]

jkratzer: I see zero reports in the bucket in FM. In the description you said detected while running CI tests and also marked this as blocks domino. Do you still see this?

Comment 3

[Jason Kratzer [:jkratzer]]

(In reply to Tyson Smith [:tsmith] from comment #2)

jkratzer: I see zero reports in the bucket in FM. In the description you said detected while running CI tests and also marked this as blocks domino. Do you still see this?

The "detected while running CI tests" is from :decoder's original TSAN bug template. Since the bucket has 0 new crashes in FM we can probably close this for now. I'll reopen it if we see it again.

Comment 4

[Tyson Smith [:tsmith]]

Attachment: log.txt

We hit this while fuzzing and looks like we got a better trace and a reproducible test case.

The new log has both write stacks and I will attache the test case once it is reduced.

Comment 5

[Tyson Smith [:tsmith]]

Attachment: testcase.html