Re: [PATCH 15/34] rcu: Add noinstr-fast rcu_read_{,un}lock_tasks_trace() APIs

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 23, 2025 at 07:20:17AM -0700, Paul E. McKenney wrote:
> When expressing RCU Tasks Trace in terms of SRCU-fast, it was
> necessary to keep a nesting count and per-CPU srcu_ctr structure
> pointer in the task_struct structure, which is slow to access.
> But an alternative is to instead make rcu_read_lock_tasks_trace() and
> rcu_read_unlock_tasks_trace(), which match the underlying SRCU-fast
> semantics, avoiding the task_struct accesses.
> 
> When all callers have switched to the new API, the previous
> rcu_read_lock_trace() and rcu_read_unlock_trace() APIs will be removed.
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: <bpf@vger.kernel.org>
> ---
>  include/linux/rcupdate_trace.h | 37 ++++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
> index 0bd47f12ecd17b..b87151e6b23881 100644
> --- a/include/linux/rcupdate_trace.h
> +++ b/include/linux/rcupdate_trace.h
> @@ -34,6 +34,43 @@ static inline int rcu_read_lock_trace_held(void)
>  
>  #ifdef CONFIG_TASKS_TRACE_RCU
>  
> +/**
> + * rcu_read_lock_tasks_trace - mark beginning of RCU-trace read-side critical section
> + *
> + * When synchronize_rcu_tasks_trace() is invoked by one task, then that
> + * task is guaranteed to block until all other tasks exit their read-side
> + * critical sections.  Similarly, if call_rcu_trace() is invoked on one
> + * task while other tasks are within RCU read-side critical sections,
> + * invocation of the corresponding RCU callback is deferred until after
> + * the all the other tasks exit their critical sections.
> + *
> + * For more details, please see the documentation for srcu_read_lock_fast().
> + */
> +static inline struct srcu_ctr __percpu *rcu_read_lock_tasks_trace(void)
> +{
> +	struct srcu_ctr __percpu *ret = srcu_read_lock_fast(&rcu_tasks_trace_srcu_struct);
> +
> +	if (IS_ENABLED(CONFIG_ARCH_WANTS_NO_INSTR))
> +		smp_mb();

I am somewhat confused by the relation between noinstr and smp_mb()
here. Subject mentions is, but Changelog is awfully silent again.

Furthermore I note that this is a positive while unlock is a negative
relation between the two. Which adds even more confusion.

> +	return ret;
> +}
> +
> +/**
> + * rcu_read_unlock_tasks_trace - mark end of RCU-trace read-side critical section
> + * @scp: return value from corresponding rcu_read_lock_tasks_trace().
> + *
> + * Pairs with the preceding call to rcu_read_lock_tasks_trace() that
> + * returned the value passed in via scp.
> + *
> + * For more details, please see the documentation for rcu_read_unlock().
> + */
> +static inline void rcu_read_unlock_tasks_trace(struct srcu_ctr __percpu *scp)
> +{
> +	if (!IS_ENABLED(CONFIG_ARCH_WANTS_NO_INSTR))
> +		smp_mb();
> +	srcu_read_unlock_fast(&rcu_tasks_trace_srcu_struct, scp);
> +}
> +
>  /**
>   * rcu_read_lock_trace - mark beginning of RCU-trace read-side critical section
>   *
> -- 
> 2.40.1
>