Re: [PATCH bpf-next] bpf: Fix deadlock between rcu_tasks_trace and event_mutex.

[Hou Tao <houtao@huaweicloud.com>]

Hi,

On 2/25/2025 6:16 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
>
> Fix the following deadlock:
> CPU A
> _free_event()
>   perf_kprobe_destroy()
>     mutex_lock(&event_mutex)
>       perf_trace_event_unreg()
>         synchronize_rcu_tasks_trace()
>
> There are several paths where _free_event() grabs event_mutex
> and calls sync_rcu_tasks_trace. Above is one such case.
>
> CPU B
> bpf_prog_test_run_syscall()
>   rcu_read_lock_trace()
>     bpf_prog_run_pin_on_cpu()
>       bpf_prog_load()
>         bpf_tracing_func_proto()
>           trace_set_clr_event()
>             mutex_lock(&event_mutex)

Considering the unregistered case is not so frequency, would it better
to use mutex_trylock firstly, then fallback to workqueue when the
event_mutex is busy ?
>
> Delegate trace_set_clr_event() to workqueue to avoid
> such lock dependency.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> ---
>  kernel/trace/bpf_trace.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index a612f6f182e5..13bef2462e94 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -392,7 +392,7 @@ static const struct bpf_func_proto bpf_trace_printk_proto = {
>  	.arg2_type	= ARG_CONST_SIZE,
>  };
>  
> -static void __set_printk_clr_event(void)
> +static void __set_printk_clr_event(struct work_struct *work)
>  {
>  	/*
>  	 * This program might be calling bpf_trace_printk,
> @@ -405,10 +405,11 @@ static void __set_printk_clr_event(void)
>  	if (trace_set_clr_event("bpf_trace", "bpf_trace_printk", 1))
>  		pr_warn_ratelimited("could not enable bpf_trace_printk events");
>  }
> +static DECLARE_WORK(set_printk_work, __set_printk_clr_event);
>  
>  const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
>  {
> -	__set_printk_clr_event();
> +	schedule_work(&set_printk_work);
>  	return &bpf_trace_printk_proto;
>  }
>  
> @@ -451,7 +452,7 @@ static const struct bpf_func_proto bpf_trace_vprintk_proto = {
>  
>  const struct bpf_func_proto *bpf_get_trace_vprintk_proto(void)
>  {
> -	__set_printk_clr_event();
> +	schedule_work(&set_printk_work);
>  	return &bpf_trace_vprintk_proto;
>  }
>