Re: [PATCH] rethook: Use tsk->on_cpu to check task execution state

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 8 Jun 2026 09:52:37 +0800
Tengda Wu <wutengda@huaweicloud.com> wrote:

> 
> 
> On 2026/6/5 21:43, Masami Hiramatsu wrote:
> > On Thu, 4 Jun 2026 11:34:45 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> >> On Mon, Jun 01, 2026 at 08:40:01AM +0900, Masami Hiramatsu wrote:
> >>
> >>> Peter, is it OK to drop @rq from task_on_cpu()? 
> >>
> >> Sure.
> >>
> >>> Then we can use it from rethook.
> >>
> >> Well, it is in sched/sched.h, which is an internal header, and no you
> >> cannot use that header in rethook.
> > 
> > Ah, OK. Hmm, then we should not use it. Maybe ->on_cpu is also internal
> > state?
> > 
> >>
> >> But lets step back first, what is the actual problem here, why are we
> >> looking at ->on_cpu at all?
> > 
> > Tengda, can you explain it?
> > I think you want to take a stacktrace on !current process, and
> > rethook_find_ret_addr() is rejected i the task is running state.
> > 
> > But if you can share actual situation what you need, it is
> > helpful for us to understand.
> > 
> > Thank you,
> > 
> > 
> 
> 
> Sure.
> 
> Background: We are verifying the support of live patches for functions that
> have a kretprobe. The specific verification method is as follows:
> 
> We construct a function foo() that calls bar():
> 
> void bar(void)
> {
>     for (;;) {
>         schedule();
>     }
> }
> 
> void foo(void)
> {
>     bar();
> }
> 
> A kretprobe is attached to bar():
> 
> echo 'r:rp1 bar' > /sys/kernel/tracing/kprobe_events
> echo 1 > /sys/kernel/tracing/events/kprobes/rp1/enable
> 
> Then foo() is triggered. The expected behavior is that bar() will call
> schedule() and yield the CPU.
> 
> After that, the live patch is activated to attempt replacing the implementation
> of foo(). The expectation is that this should succeed.
> 
> However, in reality, because the task that called schedule() is still in the
> RUNNING state, the condition task_is_running(tsk) inside rethook_find_ret_addr()
> is not satisfied, causing the function to return early. This, in turn,
> prevents stack_trace_save_tsk_reliable() from determining the stack as
> reliable, leading to a failure in activating the live patch.

Hmm is the bar() doing infinite loop, or limited loop but take a long time
so just yield a while? Anyway, it seems like a non-good design pattern.
Is it possible to avoid busy loops and instead use Workers, or wait for
something to complete or for input within a loop?

> 
> **Not sure if this is correct:**
> 
> We believe that after a task voluntarily calls schedule(), when the stack
> is expected to be reliable, it is a safe time to activate a live patch.

In this case, I don't know how to block the loop inside the bar.
Even if !tsk->on_cpu, the tsk can restart running right after checking
the flag.

> Additionally, a similar tsk->on_cpu check can be found elsewhere in the
> kernel (See task_on_another_cpu() in arch/x86/include/asm/unwind.h).
> Therefore, we propose changing the task_is_running(tsk) condition to
> tsk->on_cpu.

Yes, but the caller said there is another check to ensure the race.

        /*                  
         * Refuse to unwind the stack of a task while it's executing on another
         * CPU.  This check is racy, but that's ok: the unwinder has other
         * checks to prevent it from going off the rails.
         */
        if (task_on_another_cpu(task))
                goto err;

Josh, do you know how this avoid the race case?

Thank you,

> 
> Thanks,
> Tengda
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>