[PATCH -tip v2] Enable optprobe on preepmtive kernel

[Masami Hiramatsu <mhiramat@kernel.org>]

This patch enables optprobe even with CONFIG_PREEMPT. The
previous patch is https://lkml.org/lkml/2017/5/24/464 .

With Paul's hack (https://lkml.org/lkml/2017/5/25/435),
now synchronize_rcu_tasks() is translated to synchronize_sched()
when TASKS_RCU=n. So this patch just replace the
synchronize_sched() with synchronize_rcu_tasks() and
select TASKS_RCU if PREEMPT=y & OPTPROBES=y. 

Here is the reason why this change is needed.
Since the jump optimized kprobes can replace multiple
instructions, there can be tasks which are interrupted
on the 2nd (or 3rd) instructions. If the kprobe
replaces those instructions by a jump instruction,
when those tasks back to the interrupted place, it is
a middle of the jump instruction and causes a kernel
panic.

To avoid such tragedies in advance, kprobe optimizer
prepare a detour route using normal kprobe (e.g.
int3 breakpoint on x86), and wait for the tasks which
is interrrupted on such place by synchronize_sched()
when CONFIG_PREEMPT=n.

If CONFIG_PREEMPT=y, things be more complicated, because
such interrupted thread can be preempted (other thread
can be scheduled in interrupt handler.) This means we
can not ensure all tasks run in safe zone by
synchronize_sched().

However, we have synchronize_rcu_tasks() which can
ensure that all preempted tasks back on track and
scheduled. kprobes optimizer can wait for those preempted
tasks scheduled normally by synchronize_rcu_tasks().

Thank you,

---

Masami Hiramatsu (1):
      kprobes: Use synchronize_rcu_tasks() for optprobe with CONFIG_PREEMPT


 arch/Kconfig     |    2 +-
 kernel/kprobes.c |   18 +++++++++++++-----
 2 files changed, 14 insertions(+), 6 deletions(-)

--
Masami Hiramatsu <mhiramat@kernel.org>