Re: [PATCH bpf-next 1/3] bpf: Free elements after one RCU-tasks-trace grace period

[Hou Tao <houtao@huaweicloud.com>]

Hi,

On 10/14/2022 3:00 AM, Paul E. McKenney wrote:
> On Thu, Oct 13, 2022 at 09:25:31AM +0800, Hou Tao wrote:
>> Hi,
>>
>> On 10/13/2022 12:11 AM, Paul E. McKenney wrote:
>>> On Wed, Oct 12, 2022 at 05:26:26PM +0800, Hou Tao wrote:
SNIP
>>> How about if I produce a patch for rcu_trace_implies_rcu_gp() and let
>>> you carry it with your series?  That way I don't have an unused function
>>> in -rcu and you don't have to wait for me to send it upstream?
>> Sound reasonable to me. Also thanks for your suggestions.
> Here you go!  Thoughts?
It looks great and thanks for it.
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 2eac2f7a9a6d8921e8084a6acdffa595e99dbd17
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Thu Oct 13 11:54:13 2022 -0700
>
>     rcu-tasks: Provide rcu_trace_implies_rcu_gp()
>     
>     As an accident of implementation, an RCU Tasks Trace grace period also
>     acts as an RCU grace period.  However, this could change at any time.
>     This commit therefore creates an rcu_trace_implies_rcu_gp() that currently
>     returns true to codify this accident.  Code relying on this accident
>     must call this function to verify that this accident is still happening.
>     
>     Reported-by: Hou Tao <houtao@huaweicloud.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>     Cc: Alexei Starovoitov <ast@kernel.org>
>     Cc: Martin KaFai Lau <martin.lau@linux.dev>
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 08605ce7379d7..8822f06e4b40c 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -240,6 +240,18 @@ static inline void exit_tasks_rcu_start(void) { }
>  static inline void exit_tasks_rcu_finish(void) { }
>  #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
>  
> +/**
> + * rcu_trace_implies_rcu_gp - does an RCU Tasks Trace grace period imply an RCU grace period?
> + *
> + * As an accident of implementation, an RCU Tasks Trace grace period also
> + * acts as an RCU grace period.  However, this could change at any time.
> + * Code relying on this accident must call this function to verify that
> + * this accident is still happening.
> + *
> + * You have been warned!
> + */
> +static inline bool rcu_trace_implies_rcu_gp(void) { return true; }
> +
>  /**
>   * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
>   *
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index b0b885e071fa8..fe9840d90e960 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1535,6 +1535,8 @@ static void rcu_tasks_trace_postscan(struct list_head *hop)
>  {
>  	// Wait for late-stage exiting tasks to finish exiting.
>  	// These might have passed the call to exit_tasks_rcu_finish().
> +
> +	// If you remove the following line, update rcu_trace_implies_rcu_gp()!!!
>  	synchronize_rcu();
>  	// Any tasks that exit after this point will set
>  	// TRC_NEED_QS_CHECKED in ->trc_reader_special.b.need_qs.