If I accidentally launch tab completion when debugging large binary, gdb will freeze for some time scanning symbol table (up to 1 minute in my case). So I have to wait until
There are still open bugs on tab completion in gdb Bugzilla:
Edit: gdb 7.10.0 has a max-completions variable with a default of 200.
Control c should interrupt it. Have you tried that?
If it doesn't work, you may need a more recent version of gdb.