Re: [PATCH] rcu: Remove RCU_NONIDLE()

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Mar 20, 2023 at 06:37:51PM +0100, Peter Zijlstra wrote:
> 
> Since there are now exactly _zero_ users of RCU_NONIDLE(), make it go
> away before someone else decides to (ab)use it.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  .../RCU/Design/Requirements/Requirements.rst       | 36 +---------------------
>  Documentation/RCU/whatisRCU.rst                    |  1 -
>  include/linux/rcupdate.h                           | 25 ---------------
>  3 files changed, 1 insertion(+), 61 deletions(-)
> 
> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
> index 49387d823619..77155b51d4c2 100644
> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> @@ -2071,41 +2071,7 @@ call.
>  
>  Because RCU avoids interrupting idle CPUs, it is illegal to execute an
>  RCU read-side critical section on an idle CPU. (Kernels built with
> -``CONFIG_PROVE_RCU=y`` will splat if you try it.) The RCU_NONIDLE()
> -macro and ``_rcuidle`` event tracing is provided to work around this
> -restriction. In addition, rcu_is_watching() may be used to test
> -whether or not it is currently legal to run RCU read-side critical
> -sections on this CPU. I learned of the need for diagnostics on the one
> -hand and RCU_NONIDLE() on the other while inspecting idle-loop code.
> -Steven Rostedt supplied ``_rcuidle`` event tracing, which is used quite
> -heavily in the idle loop. However, there are some restrictions on the
> -code placed within RCU_NONIDLE():
> -
> -#. Blocking is prohibited. In practice, this is not a serious
> -   restriction given that idle tasks are prohibited from blocking to
> -   begin with.
> -#. Although nesting RCU_NONIDLE() is permitted, they cannot nest
> -   indefinitely deeply. However, given that they can be nested on the
> -   order of a million deep, even on 32-bit systems, this should not be a
> -   serious restriction. This nesting limit would probably be reached
> -   long after the compiler OOMed or the stack overflowed.
> -#. Any code path that enters RCU_NONIDLE() must sequence out of that
> -   same RCU_NONIDLE(). For example, the following is grossly
> -   illegal:
> -
> -      ::
> -
> -	  1     RCU_NONIDLE({
> -	  2       do_something();
> -	  3       goto bad_idea;  /* BUG!!! */
> -	  4       do_something_else();});
> -	  5   bad_idea:
> -
> -
> -   It is just as illegal to transfer control into the middle of
> -   RCU_NONIDLE()'s argument. Yes, in theory, you could transfer in
> -   as long as you also transferred out, but in practice you could also
> -   expect to get sharply worded review comments.
> +``CONFIG_PROVE_RCU=y`` will splat if you try it.) 
>  
>  It is similarly socially unacceptable to interrupt an ``nohz_full`` CPU
>  running in userspace. RCU must therefore track ``nohz_full`` userspace
> diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
> index 2c5563a91998..c3b1cbfa1530 100644
> --- a/Documentation/RCU/whatisRCU.rst
> +++ b/Documentation/RCU/whatisRCU.rst
> @@ -1117,7 +1117,6 @@ in docbook.  Here is the list, by category.
>  
>  	RCU_LOCKDEP_WARN
>  	rcu_sleep_check
> -	RCU_NONIDLE
>  
>  All: Unchecked RCU-protected pointer access::
>  
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 094321c17e48..ddd42efc6224 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -156,31 +156,6 @@ static inline int rcu_nocb_cpu_deoffload(int cpu) { return 0; }
>  static inline void rcu_nocb_flush_deferred_wakeup(void) { }
>  #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
>  
> -/**
> - * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
> - * @a: Code that RCU needs to pay attention to.
> - *
> - * RCU read-side critical sections are forbidden in the inner idle loop,
> - * that is, between the ct_idle_enter() and the ct_idle_exit() -- RCU
> - * will happily ignore any such read-side critical sections.  However,
> - * things like powertop need tracepoints in the inner idle loop.
> - *
> - * This macro provides the way out:  RCU_NONIDLE(do_something_with_RCU())
> - * will tell RCU that it needs to pay attention, invoke its argument
> - * (in this example, calling the do_something_with_RCU() function),
> - * and then tell RCU to go back to ignoring this CPU.  It is permissible
> - * to nest RCU_NONIDLE() wrappers, but not indefinitely (but the limit is
> - * on the order of a million or so, even on 32-bit systems).  It is
> - * not legal to block within RCU_NONIDLE(), nor is it permissible to
> - * transfer control either into or out of RCU_NONIDLE()'s statement.
> - */
> -#define RCU_NONIDLE(a) \
> -	do { \
> -		ct_irq_enter_irqson(); \
> -		do { a; } while (0); \
> -		ct_irq_exit_irqson(); \
> -	} while (0)
> -
>  /*
>   * Note a quasi-voluntary context switch for RCU-tasks's benefit.
>   * This is a macro rather than an inline function to avoid #include hell.