Debug session with gdb:
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[vstinner@buildbot-freebsd ~/cpython]$ gdb -args ./python x.py 
GNU gdb 6.1.1 [FreeBSD]
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This GDB was configured as "i386-marcel-freebsd"...run

(gdb) run
Starting program: /usr/home/vstinner/cpython/python x.py
warning: Unable to get location for thread creation breakpoint: generic error
[New LWP 100106]
[New Thread 0x81f8000 (LWP 100089)]

Program received signal SIGBUS, Bus error.
[Switching to Thread 0x81f8000 (LWP 100083)]
stack_overflow (min_sp=0xb97fe7f0, max_sp=0xc5ffe7f0, depth=0xbfbfe7f0)
    at ./Modules/faulthandler.c:870
870	    buffer[0] = 1;
(gdb) signal SIGBUS
Continuing with signal SIGBUS.
[New LWP 100083]

Program received signal SIGBUS, Bus error.
[Switching to LWP 100083]
0x2824df2a in signalcontext () from /lib/libc.so.6
(gdb) handle SIGBUS nostop
Signal        Stop	Print	Pass to program	Description
SIGBUS        No	Yes	Yes		Bus error
(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /usr/home/vstinner/cpython/python x.py
warning: Unable to get location for thread creation breakpoint: generic error
[New LWP 100096]
[New Thread 0x81f8000 (LWP 100096)]

Program received signal SIGBUS, Bus error.
[New LWP 100096]

Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
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If I revert the commit, faulthandler is able to dump the traceback on a stack overflow. But if I create a thread (and wait until it exits), faulthandler signal handler (for SIGBUS) is no more called.