https://android.googlesource.com/platform/bionic/#host-tests
https://android.googlesource.com/platform/bionic/+/master/libc/bionic/libc_init_common.cpp

When running certain multi threaded program in 32bit containers, the PIDs can balloon and easily exceed libc's limit.
The following error will be shown:

Limited by the size of pthread_mutex_t, 32 bit bionic libc only accepts pid <= 65535, but current pid is 65969
libc: Limited by the size of pthread_mutex_t, 32 bit bionic libc only accepts pid <= 65535, but current pid is 65969
libc: Fatal signal 6 (SIGABRT), code -6 (SI_TKILL) in tid 65969 (emcc), pid 65969 (emcc)

This was spotted when running emscripten tests in i686 container on x86_64 host
which the pid_max value of both host and container is

$ cat /proc/sys/kernel/pid_max 
4194304

The only way to fix this is to set /proc/sys/kernel/pid_max to 65535 but setting this will affect both container and host globally at runtime. I think we can list this as a caveat.

EDIT: updated to 65535 in 6189bbd. Sorry for the confusion future readers.