The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision 12fb5e522dd3.
It looks like we are giving up a reference to a
nsHtml5OwningUTF16Buffer from to threads at the same time. Since this is a non-threadsafe
RefPtr this can lead to various side-effects. For this particular race, where both threads are decreasing the non-atomic reference count, one of the updates might be trashed, so we end up with the reference count only being decremented once and have a memory leak. However, If this
RefPtr is touched from two different threads at the same time, then there is likely some underlying ownership problem and if we increased the reference count on one thread while releasing on the other, it would cause a use-after-free.
I'm filing this as a security bug as a start until this has been fully investigated.
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.