The attached crash information was detected with ThreadSanitizer on mozilla-central revision 20200428-a99c73301874.

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.

Jason, I see the test case uses getUserMedia, but none of the code in the stacks is media code, I'm redirecting this.

I see there's a patch to fix a race in the interposer in bug 1630655, but I don't know if this is the same thing or not.

I think it's very likely the same thing, but of course without the stack for the read it's hard to say. Do you want me to land just that patch? Bug 1630655 might have to land after the code freeze.

(In reply to Doug Thayer [:dthayer] from comment #4)

I think it's very likely the same thing, but of course without the stack for the read it's hard to say. Do you want me to land just that patch? Bug 1630655 might have to land after the code freeze.

Sure, if you could land it separately to fix up a TSan race that would be nice.

Because this bug's Severity has not been changed from the default since it was filed, and it's Priority is -- (non,) indicating it has has not been previously triaged, the bug's Severity is being updated to -- (default, untriaged.)

Looks like this was fixed a long time ago.