Re: [RFC] sched,livepatch: call stop_one_cpu in klp_check_and_switch_task

[Song Liu <songliubraving@fb.com>]

> On May 9, 2022, at 8:52 AM, Rik van Riel <riel@surriel.com> wrote:
> 
> After talking with Peter, this seems like it might be a potential approach
> to fix the issue for kernels both with PREEMPT enabled and disabled.
> 
> If this looks like a reasonable approach to people, we can run experiments
> with this patch on a few thousand systems, and compare it with the kernel
> live patch transition latencies (and number of failures) on a kernel without
> that patch.
> 
> Does this look like an approach that could work?

Hi Petr, 

IIUC, this is similar to you proposal. Could you please share your thoughts
on this? 

Thanks,
Song


> 
> ---8<---
> sched,livepatch: call stop_one_cpu in klp_check_and_switch_task
> 
> If a running task fails to transition to the new kernel live patch after the
> first attempt, use the stopper thread to preempt it during subsequent attempts
> at switching to the new kernel live patch.
> 
> <INSERT EXPERIMENTAL RESULTS HERE>
> 
> Signed-off-by: Rik van Riel <riel@surriel.com>
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index 5d03a2ad1066..26e9e5f09822 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -9,6 +9,7 @@
> 
> #include <linux/cpu.h>
> #include <linux/stacktrace.h>
> +#include <linux/stop_machine.h>
> #include "core.h"
> #include "patch.h"
> #include "transition.h"
> @@ -281,6 +282,11 @@ static int klp_check_and_switch_task(struct task_struct *task, void *arg)
> 	return 0;
> }
> 
> +static int kpatch_dummy_fn(void *dummy)
> +{
> +	return 0;
> +}
> +
> /*
>  * Try to safely switch a task to the target patch state.  If it's currently
>  * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or
> @@ -315,6 +321,9 @@ static bool klp_try_switch_task(struct task_struct *task)
> 	case -EBUSY:	/* klp_check_and_switch_task() */
> 		pr_debug("%s: %s:%d is running\n",
> 			 __func__, task->comm, task->pid);
> +		/* Preempt the task from the second KLP switch attempt. */
> +		if (klp_signals_cnt)
> +			stop_one_cpu(task_cpu(task), kpatch_dummy_fn, NULL);
> 		break;
> 	case -EINVAL:	/* klp_check_and_switch_task() */
> 		pr_debug("%s: %s:%d has an unreliable stack\n",
>