Created attachment 8370441 [details] Logfile with TSan trace The attached logfile shows a thread/data race (mozilla-central revision 44ba69cacd7e) detected by TSan (ThreadSanitizer). Typically, races reported by TSan are not false positives, but it is possible that the race is benign. Even in this case though, we should try to come up with a fix unless this would cause inacceptable performance issues. Also note that seemingly benign races can possibly be harmful (also depending on the compiler and the architecture) . If the bug cannot be fixed, then this bug should be used to either make a compile-time annotation for blacklisting or add an entry to the runtime blacklist.  http://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
This looks like a PJS Ion IC race.
This is a known race, and it has been addressed during the review process. We paid attention at keeping this race aligned on a cache line and we should have assertions about it.
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