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References: <20240407090558.3395-1-jiangshanlai@gmail.com> <20240407090558.3395-12-jiangshanlai@gmail.com>
In-Reply-To: <20240407090558.3395-12-jiangshanlai@gmail.com>
From: Joel Fernandes <joel@joelfernandes.org>
Date: Tue, 23 Apr 2024 13:26:06 -0400
Message-ID: <CAEXW_YTin=EgjoypCD=eYh-NLRwxbhdr+B-jBO5TpC4qcef2_Q@mail.gmail.com>
Subject: Re: [PATCH V2 11/11] x86/rcu: Add THUNK rcu_read_unlock_special_thunk
To: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, x86@kernel.org, 
	Lai Jiangshan <jiangshan.ljs@antgroup.com>, "Paul E. McKenney" <paulmck@kernel.org>, 
	Peter Zijlstra <peterz@infradead.org>, Frederic Weisbecker <frederic@kernel.org>, 
	Andy Lutomirski <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, 
	"H. Peter Anvin" <hpa@zytor.com>
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Hello Lai,

On Sun, Apr 7, 2024 at 5:07=E2=80=AFAM Lai Jiangshan <jiangshanlai@gmail.co=
m> wrote:
>
> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
>
> 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@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> ---
>  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/rc=
u_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