[PATCH V2 10/11] x86/rcu: Add rcu_preempt_count

[Lai Jiangshan]

From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>

Implement PCPU_RCU_PREEMPT_COUNT for x86.
Mainly copied from asm/preempt.h

Make rcu_read_[un]lock() inlined for rcu-preempt.
Make rcu_read_lock() only one instruction.
Make rcu_read_unlock() only two instructions in the fast path.

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/Kconfig                   |   1 +
 arch/x86/include/asm/current.h     |   3 +
 arch/x86/include/asm/rcu_preempt.h | 107 +++++++++++++++++++++++++++++
 arch/x86/kernel/cpu/common.c       |   4 ++
 4 files changed, 115 insertions(+)
 create mode 100644 arch/x86/include/asm/rcu_preempt.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4fff6ed46e90..e805cac3763d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -256,6 +256,7 @@ config X86
 	select HAVE_OBJTOOL			if X86_64
 	select HAVE_OPTPROBES
 	select HAVE_PAGE_SIZE_4KB
+	select HAVE_PCPU_RCU_PREEMPT_COUNT
 	select HAVE_PCSPKR_PLATFORM
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_EVENTS_NMI
diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
index bf5953883ec3..dcc2ef784120 100644
--- a/arch/x86/include/asm/current.h
+++ b/arch/x86/include/asm/current.h
@@ -24,6 +24,9 @@ struct pcpu_hot {
 			unsigned long		top_of_stack;
 			void			*hardirq_stack_ptr;
 			u16			softirq_pending;
+#ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+			int			rcu_preempt_count;
+#endif // #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
 #ifdef CONFIG_X86_64
 			bool			hardirq_stack_inuse;
 #else
diff --git a/arch/x86/include/asm/rcu_preempt.h b/arch/x86/include/asm/rcu_preempt.h
new file mode 100644
index 000000000000..cb25ebe038a5
--- /dev/null
+++ b/arch/x86/include/asm/rcu_preempt.h
@@ -0,0 +1,107 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_RCU_PREEMPT_H
+#define __ASM_RCU_PREEMPT_H
+
+#include <asm/rmwcc.h>
+#include <asm/percpu.h>
+#include <asm/current.h>
+
+#ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+
+/* We use the MSB mostly because its available */
+#define RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED	0x80000000
+
+/*
+ * We use the RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED bit as an inverted
+ * current->rcu_read_unlock_special.s such that a decrement hitting 0
+ * means we can and should call rcu_read_unlock_special().
+ */
+#define RCU_PREEMPT_INIT	(0 + RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED)
+
+/*
+ * We mask the RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED bit so as not to
+ * confuse all current users that think a non-zero value indicates we
+ * are in a critical section.
+ */
+static inline int pcpu_rcu_preempt_count(void)
+{
+	return raw_cpu_read_4(pcpu_hot.rcu_preempt_count) & ~RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED;
+}
+
+static inline void pcpu_rcu_preempt_count_set(int count)
+{
+	int old, new;
+
+	old = raw_cpu_read_4(pcpu_hot.rcu_preempt_count);
+	do {
+		new = (old & RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED) |
+			(count & ~RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+	} while (!raw_cpu_try_cmpxchg_4(pcpu_hot.rcu_preempt_count, &old, new));
+}
+
+/*
+ * We fold the RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED bit into the RCU
+ * preempt count such that rcu_read_unlock() can decrement and test for
+ * the need of unlock-special handling with a single instruction.
+ *
+ * We invert the actual bit, so that when the decrement hits 0 we know
+ * we both reach a quiescent state (no rcu preempt count) and need to
+ * handle unlock-special (the bit is cleared), normally to report the
+ * quiescent state immediately.
+ */
+
+static inline void pcpu_rcu_preempt_special_set(void)
+{
+	raw_cpu_and_4(pcpu_hot.rcu_preempt_count, ~RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+}
+
+static inline void pcpu_rcu_preempt_special_clear(void)
+{
+	raw_cpu_or_4(pcpu_hot.rcu_preempt_count, RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+}
+
+static inline bool pcpu_rcu_preempt_special_test(void)
+{
+	return !(raw_cpu_read_4(pcpu_hot.rcu_preempt_count) & RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+}
+
+static inline void pcpu_rcu_preempt_switch(int count, bool special)
+{
+	if (likely(!special))
+		raw_cpu_write(pcpu_hot.rcu_preempt_count, count | RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED);
+	else
+		raw_cpu_write(pcpu_hot.rcu_preempt_count, count);
+}
+
+/*
+ * The various rcu_preempt_count add/sub methods
+ */
+
+static __always_inline void pcpu_rcu_preempt_count_add(int val)
+{
+	raw_cpu_add_4(pcpu_hot.rcu_preempt_count, val);
+}
+
+static __always_inline void pcpu_rcu_preempt_count_sub(int val)
+{
+	raw_cpu_add_4(pcpu_hot.rcu_preempt_count, -val);
+}
+
+/*
+ * Because we keep RCU_PREEMPT_UNLOCK_SPECIAL_INVERTED set when we do
+ * _not_ need to handle unlock-special for a fast-path decrement.
+ */
+static __always_inline bool pcpu_rcu_preempt_count_dec_and_test(void)
+{
+	return GEN_UNARY_RMWcc("decl", __my_cpu_var(pcpu_hot.rcu_preempt_count), e,
+			       __percpu_arg([var]));
+}
+
+#define pcpu_rcu_read_unlock_special()						\
+do {										\
+	rcu_read_unlock_special();						\
+} while (0)
+
+#endif // #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+
+#endif /* __ASM_RCU_PREEMPT_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 5c1e6d6be267..918b1f5cb75d 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1995,6 +1995,10 @@ DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
 	.current_task	= &init_task,
 	.preempt_count	= INIT_PREEMPT_COUNT,
 	.top_of_stack	= TOP_OF_INIT_STACK,
+
+#ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
+	.rcu_preempt_count	= RCU_PREEMPT_INIT,
+#endif // #ifdef CONFIG_PCPU_RCU_PREEMPT_COUNT
 };
 EXPORT_PER_CPU_SYMBOL(pcpu_hot);
 EXPORT_PER_CPU_SYMBOL(const_pcpu_hot);
-- 
2.19.1.6.gb485710b