Re: [PATCH v2] kprobes: Use synchronize_rcu_tasks_rude in kprobe_optimizer

[Steven Rostedt]

On Thu, 18 Jan 2024 02:18:42 +0000
Chen Zhongjin <chenzhongjin@xxxxxxxxxx> wrote:

> There is a deadlock scenario in kprobe_optimizer():
> 
> pid A				pid B			pid C
> kprobe_optimizer()		do_exit()		perf_kprobe_init()
> mutex_lock(&kprobe_mutex)	exit_tasks_rcu_start()	mutex_lock(&kprobe_mutex)
> synchronize_rcu_tasks()		zap_pid_ns_processes()	// waiting kprobe_mutex
> // waiting tasks_rcu_exit_srcu	kernel_wait4()
> 				// waiting pid C exit
> 
> To avoid this deadlock loop, use synchronize_rcu_tasks_rude() in kprobe_optimizer()
> rather than synchronize_rcu_tasks(). synchronize_rcu_tasks_rude() can also promise
> that all preempted tasks have scheduled, but it will not wait tasks_rcu_exit_srcu.
> 

Did lockdep detect this? If not, we should fix that.

I'm also thinking if we should find another solution, as this seems more of
a work around than a fix.

> Fixes: a30b85df7d59 ("kprobes: Use synchronize_rcu_tasks() for optprobe with CONFIG_PREEMPT=y")
> Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
> ---
> v1 -> v2: Add Fixes tag
> ---
>  arch/Kconfig     | 2 +-
>  kernel/kprobes.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index f4b210ab0612..dc6a18854017 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -104,7 +104,7 @@ config STATIC_CALL_SELFTEST
>  config OPTPROBES
>  	def_bool y
>  	depends on KPROBES && HAVE_OPTPROBES
> -	select TASKS_RCU if PREEMPTION
> +	select TASKS_RUDE_RCU

Is this still a bug if PREEMPTION is not enabled?

-- Steve

>  
>  config KPROBES_ON_FTRACE
>  	def_bool y
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index d5a0ee40bf66..09056ae50c58 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -623,7 +623,7 @@ static void kprobe_optimizer(struct work_struct *work)
>  	 * Note that on non-preemptive kernel, this is transparently converted
>  	 * to synchronoze_sched() to wait for all interrupts to have completed.
>  	 */
> -	synchronize_rcu_tasks();
> +	synchronize_rcu_tasks_rude();
>  
>  	/* Step 3: Optimize kprobes after quiesence period */
>  	do_optimize_kprobes();