[PATCH 00/16] ptrace: cleanups and calling do_cldstop with only siglock

Eric W. Biederman
2022-05-18T22:49:50Z

For ptrace_stop to work on PREEMT_RT no spinlocks can be taken once
ptrace_freeze_traced has completed successfully. Which fundamentally
means the lock dance of dropping siglock and grabbing tasklist_lock does
not work on PREEMPT_RT. So I have worked through what is necessary so
that tasklist_lock does not need to be grabbed in ptrace_stop after
siglock is dropped.

I have explored several alternate ways of getting there and along the
way I found a lot of small bug fixes/cleanups that don't necessarily
contribute to the final result but that or worthwhile on their own. So
I have included those changes in this set of changes just so they don't
get lost.

In addition I had a conversation with Thomas Gleixner recently that
emphasized for me the need to reduce the hold times of tasklist_lock,
and that made me realize that in principle it is possible.
https://lkml.kernel.org/r/87mtfmhap2.fsf@email.froward.int.ebiederm.org

Which is a long way of saying that not taking tasklist_lock in
ptrace_stop is good not just for PREMPT_RT but also for improving the
scalability of the kernel in general.

After this set of changes only cgroup_enter_frozen should remain a
stumbling block for PREEMPT_RT in the ptrace_stop path.

Eric W. Biederman (16):
signal/alpha: Remove unused definition of TASK_REAL_PARENT
signal/ia64: Remove unused definition of IA64_TASK_REAL_PARENT_OFFSET
kdb: Use real_parent when displaying a list of processes
powerpc/xmon: Use real_parent when displaying a list of processes
ptrace: Remove dead code from __ptrace_detach
ptrace: Remove unnecessary locking in ptrace_(get|set)siginfo
signal: Wake up the designated parent
ptrace: Only populate last_siginfo from ptrace
ptrace: In ptrace_setsiginfo deal with invalid si_signo
ptrace: In ptrace_signal look at what the debugger did with siginfo
ptrace: Use si_sino as the signal number to resume with
ptrace: Stop protecting ptrace_set_signr with tasklist_lock
ptrace: Document why ptrace_setoptions does not need a lock
signal: Protect parent child relationships by childs siglock
ptrace: Use siglock instead of tasklist_lock in ptrace_check_attach
signal: Always call do_notify_parent_cldstop with siglock held

arch/alpha/kernel/asm-offsets.c | 1 -
arch/ia64/kernel/asm-offsets.c | 1 -
arch/powerpc/xmon/xmon.c | 2 +-
kernel/debug/kdb/kdb_main.c | 2 +-
kernel/exit.c | 23 +++-
kernel/fork.c | 12 +-
kernel/ptrace.c | 132 ++++++++----------
kernel/signal.c | 296 ++++++++++++++++++++++++++--------------
8 files changed, 279 insertions(+), 190 deletions(-)

Eric