[Bug 250954] ptrace(): weird ordering between inheriting debug registers and reporting a new thread

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=250954

--- Comment #8 from Konstantin Belousov <kib at FreeBSD.org> ---
(In reply to Michał Górny from comment #7)
Actually not.  Or rather, I updated your second example some more to query
the list of threads on each stop, and for me it behaves exactly as I would
expect it to behave:
wait: pid=38618, waited=38618, ret=117f
threads: 100447
tid = 100447, SIGSTOP
set DR0=0x601878 (&g_val) on tid=100447
wait: pid=38618, waited=38618, ret=117f
threads: 100447 100820
tid = 100447, SIGSTOP
set DR0=0x601874 (&g_val2) on tid=100447
wait: pid=38618, waited=38618, ret=57f
threads: 100447 100820
tid = 100820, SIGTRAP w/ PL_FLAG_BORN
dr0=0x601878, g_val=0x601878, g_val2=0x601874
wait: pid=38618, waited=38618, ret=57f
threads: 100447 100820
tid = 100820, SIGTRAP w/ PL_FLAG_EXITED
thread started
thread joined
wait: pid=38618, waited=38618, ret=0
Now you set DR0 using explicit tid, and second PT_SETDBREGS only affects the
main thread, while new thread 100820 inherits initial DR0.

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