[cribbing from decoder's script, hopefully he won't mind] The attached logfile shows a thread/data race 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 unacceptable performance issues. Also note that seemingly benign races can possibly be harmful (also depending on the compiler and the architecture) [1]. 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. Looks like we can set JSRuntime::numExclusiveThreads on multiple threads... [1] http://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong

This looks related to parsing off the main thread.