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

Looks like a race on gcState_ but I am not sure why the second stack doesn't give an exact location in ShouldMarkCrossCompartment.

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.

This is a race between changing a zone's GC state while sweeping on the main thread and reading it while doing off thread marking.

The state change in endSweepingSweepGroup() is from the sweeping state to the finished state, and the check on the other thread is whether the state is the marking state or not, so this isn't definitely a problem. We should probably wait for off thread marking to finish before changing the zone state in endSweepingSweepGroup() though just to be sure.

Yoshi, do you want to have a go at this? I think we just need to factor out the code to join the sweep marking task and check the result from GCRuntime::endMarkingSweepGroup and markGrayReferencesInCurrentGroup and also call this in endSweepingSweepGroup.

sure, taking this bug

https://hg.mozilla.org/mozilla-central/rev/d4ac3919864b