The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision 6989fcd6bab3.
I've investigated this one a bit and the cause is similar to bug 1601940: There are several bool flags declared as bitfields here:
Writing any of these on one thread while reading/writing another of these on a different thread is a race because the compiler optimizes reads/writes of these fields into larger units. This should be harmless as long as we have a read/write race, but it can be very harmful as soon as we have a write/write race (two threads wanting to update different fields).
The race reported here is a read/write race so probably benign but it is very easy to turn this into a real problem (because it is not intuitive that these fields cannot be updated on different threads without synchronization, even when writing different fields).
I'm not sure if we have any code already that could cause a write/write race here. Andrew, what do you think?
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.
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 , 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) .
 One major exception is the involvement of uninstrumented code from third-party libraries.
 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.