jemalloc also has its own funny problem with threads - if you have a multi-threaded application that uses jemalloc on all threads except the main thread, then the cleanup that jemalloc runs on main thread exit will segfault. In $dayjob we use jemalloc as a sub-allocator in specific arenas. (*) The application itself is fine in production because it allocates from the main thread too, but the unit test framework only runs tests in spawned threads and the main thread of the test binary just orchestrates them. So the test binary triggers this segfault reliably.
(*): The application uses libc malloc normally, but at some places it allocates pages using `mmap(non_anonymous_tempfile)` and then uses jemalloc to partition them. jemalloc has a feature called "extent hooks" where you can customize how jemalloc gets underlying pages for its allocations, which we use to give it pages via such mmap's. Then the higher layers of the code that just want to allocate don't have to care whether those allocations came from libc malloc or mmap-backed disk file.
( https://github.com/jemalloc/jemalloc/issues/1317 Unlike what the title says, it's not Windows-specific.)
(*): The application uses libc malloc normally, but at some places it allocates pages using `mmap(non_anonymous_tempfile)` and then uses jemalloc to partition them. jemalloc has a feature called "extent hooks" where you can customize how jemalloc gets underlying pages for its allocations, which we use to give it pages via such mmap's. Then the higher layers of the code that just want to allocate don't have to care whether those allocations came from libc malloc or mmap-backed disk file.