Re: [PATCH V2 11/11] x86/rcu: Add THUNK rcu_read_unlock_special_thunk

[Joel Fernandes]

Hello Lai,

On Sun, Apr 7, 2024 at 5:07 AM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
>
> Add rcu_read_unlock_special_thunk(), so that the inlined rcu_read_unlock()
> doesn't need any code to save the caller-saved registers.
>
> Make rcu_read_unlock() only two instructions in the slow path at the
> caller site.
>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
> ---
>  arch/x86/entry/thunk.S             | 5 +++++
>  arch/x86/include/asm/rcu_preempt.h | 4 +++-
>  2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/thunk.S b/arch/x86/entry/thunk.S
> index 119ebdc3d362..10c60369a67c 100644
> --- a/arch/x86/entry/thunk.S
> +++ b/arch/x86/entry/thunk.S
> @@ -13,3 +13,8 @@ THUNK preempt_schedule_thunk, preempt_schedule
>  THUNK preempt_schedule_notrace_thunk, preempt_schedule_notrace
>  EXPORT_SYMBOL(preempt_schedule_thunk)
>  EXPORT_SYMBOL(preempt_schedule_notrace_thunk)
> +
> +#ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
> +THUNK rcu_read_unlock_special_thunk, rcu_read_unlock_special
> +EXPORT_SYMBOL_GPL(rcu_read_unlock_special_thunk)
> +#endif /* #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT */
> diff --git a/arch/x86/include/asm/rcu_preempt.h b/arch/x86/include/asm/rcu_preempt.h
> index cb25ebe038a5..acdd73b74c05 100644
> --- a/arch/x86/include/asm/rcu_preempt.h
> +++ b/arch/x86/include/asm/rcu_preempt.h
> @@ -97,9 +97,11 @@ static __always_inline bool pcpu_rcu_preempt_count_dec_and_test(void)
>                                __percpu_arg([var]));
>  }
>
> +extern asmlinkage void rcu_read_unlock_special_thunk(void);
> +
>  #define pcpu_rcu_read_unlock_special()                                         \
>  do {                                                                           \
> -       rcu_read_unlock_special();

Instead, can you not use __no_caller_saved_registers attribute for
definition of rcu_read_unlock_special() or does that not work for what
you're trying to do here?

Thanks,

 - Joel