The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision d1ac49b9eb3e.
Quick Analysis: It looks like there is a race updating the bits in
xpcom::gInitCalled vs. reading them. As
gInitCalled is not atomic, this could cause the respective one-time initializer function in
CallInitFunc to run twice. Depending on which of the functions is run twice, this could cause problems or be benign.
In this particular case, the function that could be called twice is
nsNetStartup(), which calls
nsStandardURL::InitGlobalObjects(), which is also not race-free according to
Normally I would recommend making
gInitCalled atomic, but this looks like a hot code path and we should probably figure out first if this would cause a performance regression (that said,
Atomic<bool, Relaxed> has nearly no performance overhead in reality).
In a separate Necko bug, we should probably also make
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.