Q:

GDB does not see any threads besides

GDB does not see any threads besides the one in which crash occurred; or SIGTRAP kills my program when I set a breakpoint. 

A:

This frequently happen on Linux, especially on embedded targets.  There are two common causes:

you are using glibc, and you have stripped libpthread.so.0
mismatch between libpthread.so.0 and libthread_db.so.1

GDB itself does not know how to decode "thread control blocks" maintained by glibc and considered to be glibc private implementation detail.  It uses libthread_db.so.1 (part of glibc) to help it do so.  Therefore, libthread_db.so.1 and libpthread.so.0 must match in version and compilation flags.  In addition, libthread_db.so.1 requires certain non-global symbols to be present in libpthread.so.0.  
Solution: use strip --strip-debug libpthread.so.0 instead of strip libpthread.so.0. 
If you are doing remote debugging, make sure libpthread.so.0 on target and libthread_db.so.1 on host match.