Messages in this thread | | | Date | Thu, 9 Apr 2020 23:16:32 +0900 | From | Masami Hiramatsu <> | Subject | Re: [RFC] kretprobe: Prevent triggering kretprobe from within kprobe_flush_task |
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On Thu, 9 Apr 2020 14:52:13 +0200 Jiri Olsa <jolsa@redhat.com> wrote:
> On Thu, Apr 09, 2020 at 09:38:06PM +0900, Masami Hiramatsu wrote: > > Hi Jiri, > > > > On Wed, 8 Apr 2020 18:46:41 +0200 > > Jiri Olsa <jolsa@kernel.org> wrote: > > > > > hi, > > > Ziqian reported lockup when adding retprobe on _raw_spin_lock_irqsave. > > > > Hmm, kprobe is lockless, but kretprobe involves spinlock. > > Thus, eventually, I will blacklist the _raw_spin_lock_irqsave() > > for kretprobe. > > I thought of blacklisting, but we were using that kretprobe > in a bcc script.. it's not overloaded by using bpf trampolines, > but still might be useful > > SNIP > > > > The code within the kretprobe handler checks for probe reentrancy, > > > so we won't trigger any _raw_spin_lock_irqsave probe in there. > > > > > > The problem is in outside kprobe_flush_task, where we call: > > > > > > kprobe_flush_task > > > kretprobe_table_lock > > > raw_spin_lock_irqsave > > > _raw_spin_lock_irqsave > > > > > > where _raw_spin_lock_irqsave triggers the kretprobe and installs > > > kretprobe_trampoline handler on _raw_spin_lock_irqsave return. > > > > Hmm, OK. In this case, I think we should mark this process is > > going to die and never try to kretprobe on it. > > > > > > > > The kretprobe_trampoline handler is then executed with already > > > locked kretprobe_table_locks, and first thing it does is to > > > lock kretprobe_table_locks ;-) the whole lockup path like: > > > > > > kprobe_flush_task > > > kretprobe_table_lock > > > raw_spin_lock_irqsave > > > _raw_spin_lock_irqsave ---> probe triggered, kretprobe_trampoline installed > > > > > > ---> kretprobe_table_locks locked > > > > > > kretprobe_trampoline > > > trampoline_handler > > > kretprobe_hash_lock(current, &head, &flags); <--- deadlock > > > > > > The change below sets current_kprobe in kprobe_flush_task, so the probe > > > recursion protection check is hit and the probe is never set. It seems > > > to fix the deadlock. > > > > > > I'm not sure this is the best fix, any ideas are welcome ;-) > > > > Hmm, this is a bit tricky to fix this issue. Of course, temporary disable > > kprobes (and kretprobe) on an area by filling current_kprobe might > > be a good idea, but it also involves other kprobes. > > > > How about let kretprobe skip the task which state == TASK_DEAD ? > > hum, isn't that considerable amount of paths (with state == TASK_DEAD) > that we would kill kprobes for? someone might want to trace it
OK, and I found that even after calling kprobe_flush_task(), it may be work because the task will not be switched. kretprobe instance will be reclaimed.
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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