linux-kernel - [PATCH V2 11/11] x86/rcu: Add THUNK rcu_read_unlock_special

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Date: Sun,  7 Apr 2024 17:05:58 +0800
From: Lai Jiangshan <jiangshanlai@...il.com>
To: linux-kernel@...r.kernel.org
Cc: rcu@...r.kernel.org,
	x86@...nel.org,
	Lai Jiangshan <jiangshan.ljs@...group.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH V2 11/11] x86/rcu: Add THUNK rcu_read_unlock_special_thunk

From: Lai Jiangshan <jiangshan.ljs@...group.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@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.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/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();						\
+	asm volatile ("call rcu_read_unlock_special_thunk" : ASM_CALL_CONSTRAINT);\
 } while (0)
 
 #endif // #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
-- 
2.19.1.6.gb485710b