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Subject: Re: [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth
To:     paulmck@kernel.org
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andi Kleen <ak@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Kees Cook <keescook@chromium.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Babu Moger <Babu.Moger@amd.com>,
        Rik van Riel <riel@surriel.com>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Jann Horn <jannh@google.com>,
        David Windsor <dwindsor@gmail.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Yuyang Du <duyuyang@gmail.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Richard Guy Briggs <rgb@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Michal Hocko <mhocko@suse.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "Dmitry V. Levin" <ldv@altlinux.org>, rcu@vger.kernel.org
References: <20191102124559.1135-1-laijs@linux.alibaba.com>
 <20191102124559.1135-8-laijs@linux.alibaba.com>
 <20191116154821.GE2865@paulmck-ThinkPad-P72>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
Message-ID: <4680b6f5-02d2-1d76-0b67-a5d3d9b36200@linux.alibaba.com>
Date:   Mon, 18 Nov 2019 10:02:50 +0800
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On 2019/11/16 11:48 下午, Paul E. McKenney wrote:
> On Sat, Nov 02, 2019 at 12:45:59PM +0000, Lai Jiangshan wrote:
>> Convert x86 to use a per-cpu rcu_preempt_depth. The reason for doing so
>> is that accessing per-cpu variables is a lot cheaper than accessing
>> task_struct or thread_info variables.
>>
>> We need to save/restore the actual rcu_preempt_depth when switch.
>> We also place the per-cpu rcu_preempt_depth close to __preempt_count
>> and current_task variable.
>>
>> Using the idea of per-cpu __preempt_count.
>>
>> No function call when using rcu_read_[un]lock().
>> Single instruction for rcu_read_lock().
>> 2 instructions for fast path of rcu_read_unlock().
>>
>> CC: Peter Zijlstra <peterz@infradead.org>
>> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> 
> Wouldn't putting RCU's nesting-depth counter in task info be just as fast,
> just as nice for #include/inlining, and a lot less per-architecture code?
> 
> Or am I missing some issue with the task-info approach?

struct thread_info itself is per-architecture definition.
All the arches would have to be touched if RCU's nesting-depth counter
is put int struct thread_info. Though the inlining functions can be
defined in include/asm-generic/ so that it serves for all arches
and X86 can have its own implementation in arch/x86/include/asm/.


> 
> 							Thanx, Paul
> 
>> ---
>>   arch/x86/Kconfig                         |  2 +
>>   arch/x86/include/asm/rcu_preempt_depth.h | 87 ++++++++++++++++++++++++
>>   arch/x86/kernel/cpu/common.c             |  7 ++
>>   arch/x86/kernel/process_32.c             |  2 +
>>   arch/x86/kernel/process_64.c             |  2 +
>>   include/linux/rcupdate.h                 | 24 +++++++
>>   init/init_task.c                         |  2 +-
>>   kernel/fork.c                            |  2 +-
>>   kernel/rcu/Kconfig                       |  3 +
>>   kernel/rcu/tree_exp.h                    |  2 +
>>   kernel/rcu/tree_plugin.h                 | 37 +++++++---
>>   11 files changed, 158 insertions(+), 12 deletions(-)
>>   create mode 100644 arch/x86/include/asm/rcu_preempt_depth.h
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index d6e1faa28c58..af9fedc0fdc4 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -18,6 +18,7 @@ config X86_32
>>   	select MODULES_USE_ELF_REL
>>   	select OLD_SIGACTION
>>   	select GENERIC_VDSO_32
>> +	select ARCH_HAVE_RCU_PREEMPT_DEEPTH
>>   
>>   config X86_64
>>   	def_bool y
>> @@ -31,6 +32,7 @@ config X86_64
>>   	select NEED_DMA_MAP_STATE
>>   	select SWIOTLB
>>   	select ARCH_HAS_SYSCALL_WRAPPER
>> +	select ARCH_HAVE_RCU_PREEMPT_DEEPTH
>>   
>>   config FORCE_DYNAMIC_FTRACE
>>   	def_bool y
>> diff --git a/arch/x86/include/asm/rcu_preempt_depth.h b/arch/x86/include/asm/rcu_preempt_depth.h
>> new file mode 100644
>> index 000000000000..88010ad59c20
>> --- /dev/null
>> +++ b/arch/x86/include/asm/rcu_preempt_depth.h
>> @@ -0,0 +1,87 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef __ASM_RCU_PREEMPT_DEPTH_H
>> +#define __ASM_RCU_PREEMPT_DEPTH_H
>> +
>> +#include <asm/rmwcc.h>
>> +#include <asm/percpu.h>
>> +
>> +#ifdef CONFIG_PREEMPT_RCU
>> +DECLARE_PER_CPU(int, __rcu_preempt_depth);
>> +
>> +/*
>> + * We use the RCU_NEED_SPECIAL bit as an inverted need_special
>> + * such that a decrement hitting 0 means we can and should do
>> + * rcu_read_unlock_special().
>> + */
>> +#define RCU_NEED_SPECIAL	0x80000000
>> +
>> +#define INIT_RCU_PREEMPT_DEPTH	(RCU_NEED_SPECIAL)
>> +
>> +/* We mask the RCU_NEED_SPECIAL bit so that it return real depth */
>> +static __always_inline int rcu_preempt_depth(void)
>> +{
>> +	return raw_cpu_read_4(__rcu_preempt_depth) & ~RCU_NEED_SPECIAL;
>> +}
>> +
>> +static __always_inline void rcu_preempt_depth_set(int pc)
>> +{
>> +	int old, new;
>> +
>> +	do {
>> +		old = raw_cpu_read_4(__rcu_preempt_depth);
>> +		new = (old & RCU_NEED_SPECIAL) |
>> +			(pc & ~RCU_NEED_SPECIAL);
>> +	} while (raw_cpu_cmpxchg_4(__rcu_preempt_depth, old, new) != old);
>> +}
>> +
>> +/*
>> + * We fold the RCU_NEED_SPECIAL bit into the rcu_preempt_depth such that
>> + * rcu_read_unlock() can decrement and test for needing to do special
>> + * with a single instruction.
>> + *
>> + * We invert the actual bit, so that when the decrement hits 0 we know
>> + * both it just exited the outmost rcu_read_lock() critical section and
>> + * we need to do specail (the bit is cleared) if it doesn't need to be
>> + * deferred.
>> + */
>> +
>> +static inline void set_rcu_preempt_need_special(void)
>> +{
>> +	raw_cpu_and_4(__rcu_preempt_depth, ~RCU_NEED_SPECIAL);
>> +}
>> +
>> +/*
>> + * irq needs to be disabled for clearing any bits of ->rcu_read_unlock_special
>> + * and calling this function. Otherwise it may clear the work done
>> + * by set_rcu_preempt_need_special() in interrupt.
>> + */
>> +static inline void clear_rcu_preempt_need_special(void)
>> +{
>> +	raw_cpu_or_4(__rcu_preempt_depth, RCU_NEED_SPECIAL);
>> +}
>> +
>> +static __always_inline void rcu_preempt_depth_inc(void)
>> +{
>> +	raw_cpu_add_4(__rcu_preempt_depth, 1);
>> +}
>> +
>> +static __always_inline bool rcu_preempt_depth_dec_and_test(void)
>> +{
>> +	return GEN_UNARY_RMWcc("decl", __rcu_preempt_depth, e, __percpu_arg([var]));
>> +}
>> +
>> +/* must be macros to avoid header recursion hell */
>> +#define save_restore_rcu_preempt_depth(prev_p, next_p) do {				\
>> +		prev_p->rcu_read_lock_nesting = this_cpu_read(__rcu_preempt_depth);	\
>> +		this_cpu_write(__rcu_preempt_depth, next_p->rcu_read_lock_nesting);	\
>> +	} while (0)
>> +
>> +#define DEFINE_PERCPU_RCU_PREEMP_DEPTH						\
>> +	DEFINE_PER_CPU(int, __rcu_preempt_depth) = INIT_RCU_PREEMPT_DEPTH;	\
>> +	EXPORT_PER_CPU_SYMBOL(__rcu_preempt_depth)
>> +#else /* #ifdef CONFIG_PREEMPT_RCU */
>> +#define save_restore_rcu_preempt_depth(prev_p, next_p) do {} while (0)
>> +#define DEFINE_PERCPU_RCU_PREEMP_DEPTH	/* empty */
>> +#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
>> +
>> +#endif /* __ASM_RCU_PREEMPT_DEPTH_H */
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index 9ae7d1bcd4f4..0151737e196c 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -46,6 +46,7 @@
>>   #include <asm/asm.h>
>>   #include <asm/bugs.h>
>>   #include <asm/cpu.h>
>> +#include <asm/rcu_preempt_depth.h>
>>   #include <asm/mce.h>
>>   #include <asm/msr.h>
>>   #include <asm/pat.h>
>> @@ -1633,6 +1634,9 @@ DEFINE_PER_CPU(unsigned int, irq_count) __visible = -1;
>>   DEFINE_PER_CPU(int, __preempt_count) = INIT_PREEMPT_COUNT;
>>   EXPORT_PER_CPU_SYMBOL(__preempt_count);
>>   
>> +/* close to __preempt_count */
>> +DEFINE_PERCPU_RCU_PREEMP_DEPTH;
>> +
>>   /* May not be marked __init: used by software suspend */
>>   void syscall_init(void)
>>   {
>> @@ -1690,6 +1694,9 @@ EXPORT_PER_CPU_SYMBOL(current_task);
>>   DEFINE_PER_CPU(int, __preempt_count) = INIT_PREEMPT_COUNT;
>>   EXPORT_PER_CPU_SYMBOL(__preempt_count);
>>   
>> +/* close to __preempt_count */
>> +DEFINE_PERCPU_RCU_PREEMP_DEPTH;
>> +
>>   /*
>>    * On x86_32, vm86 modifies tss.sp0, so sp0 isn't a reliable way to find
>>    * the top of the kernel stack.  Use an extra percpu variable to track the
>> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
>> index b8ceec4974fe..ab1f20353663 100644
>> --- a/arch/x86/kernel/process_32.c
>> +++ b/arch/x86/kernel/process_32.c
>> @@ -51,6 +51,7 @@
>>   #include <asm/cpu.h>
>>   #include <asm/syscalls.h>
>>   #include <asm/debugreg.h>
>> +#include <asm/rcu_preempt_depth.h>
>>   #include <asm/switch_to.h>
>>   #include <asm/vm86.h>
>>   #include <asm/resctrl_sched.h>
>> @@ -290,6 +291,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>>   	if (prev->gs | next->gs)
>>   		lazy_load_gs(next->gs);
>>   
>> +	save_restore_rcu_preempt_depth(prev_p, next_p);
>>   	this_cpu_write(current_task, next_p);
>>   
>>   	switch_fpu_finish(next_fpu);
>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>> index af64519b2695..2e1c6e829d30 100644
>> --- a/arch/x86/kernel/process_64.c
>> +++ b/arch/x86/kernel/process_64.c
>> @@ -50,6 +50,7 @@
>>   #include <asm/ia32.h>
>>   #include <asm/syscalls.h>
>>   #include <asm/debugreg.h>
>> +#include <asm/rcu_preempt_depth.h>
>>   #include <asm/switch_to.h>
>>   #include <asm/xen/hypervisor.h>
>>   #include <asm/vdso.h>
>> @@ -559,6 +560,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>>   
>>   	x86_fsgsbase_load(prev, next);
>>   
>> +	save_restore_rcu_preempt_depth(prev_p, next_p);
>>   	/*
>>   	 * Switch the PDA and FPU contexts.
>>   	 */
>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>> index a35daab95d14..0d2abf08b694 100644
>> --- a/include/linux/rcupdate.h
>> +++ b/include/linux/rcupdate.h
>> @@ -41,6 +41,29 @@ void synchronize_rcu(void);
>>   
>>   #ifdef CONFIG_PREEMPT_RCU
>>   
>> +#ifdef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH
>> +#include <asm/rcu_preempt_depth.h>
>> +
>> +#ifndef CONFIG_PROVE_LOCKING
>> +extern void rcu_read_unlock_special(void);
>> +
>> +static inline void __rcu_read_lock(void)
>> +{
>> +	rcu_preempt_depth_inc();
>> +}
>> +
>> +static inline void __rcu_read_unlock(void)
>> +{
>> +	if (unlikely(rcu_preempt_depth_dec_and_test()))
>> +		rcu_read_unlock_special();
>> +}
>> +#else
>> +void __rcu_read_lock(void);
>> +void __rcu_read_unlock(void);
>> +#endif
>> +
>> +#else /* #ifdef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH */
>> +#define INIT_RCU_PREEMPT_DEPTH (0)
>>   void __rcu_read_lock(void);
>>   void __rcu_read_unlock(void);
>>   
>> @@ -51,6 +74,7 @@ void __rcu_read_unlock(void);
>>    * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
>>    */
>>   #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
>> +#endif /* #else #ifdef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH */
>>   
>>   #else /* #ifdef CONFIG_PREEMPT_RCU */
>>   
>> diff --git a/init/init_task.c b/init/init_task.c
>> index 9e5cbe5eab7b..0a91e38fba37 100644
>> --- a/init/init_task.c
>> +++ b/init/init_task.c
>> @@ -130,7 +130,7 @@ struct task_struct init_task
>>   	.perf_event_list = LIST_HEAD_INIT(init_task.perf_event_list),
>>   #endif
>>   #ifdef CONFIG_PREEMPT_RCU
>> -	.rcu_read_lock_nesting = 0,
>> +	.rcu_read_lock_nesting = INIT_RCU_PREEMPT_DEPTH,
>>   	.rcu_read_unlock_special.s = 0,
>>   	.rcu_node_entry = LIST_HEAD_INIT(init_task.rcu_node_entry),
>>   	.rcu_blocked_node = NULL,
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index f9572f416126..7368d4ccb857 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1665,7 +1665,7 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
>>   static inline void rcu_copy_process(struct task_struct *p)
>>   {
>>   #ifdef CONFIG_PREEMPT_RCU
>> -	p->rcu_read_lock_nesting = 0;
>> +	p->rcu_read_lock_nesting = INIT_RCU_PREEMPT_DEPTH;
>>   	p->rcu_read_unlock_special.s = 0;
>>   	p->rcu_blocked_node = NULL;
>>   	INIT_LIST_HEAD(&p->rcu_node_entry);
>> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
>> index 1cc940fef17c..d2ecca49a1a4 100644
>> --- a/kernel/rcu/Kconfig
>> +++ b/kernel/rcu/Kconfig
>> @@ -14,6 +14,9 @@ config TREE_RCU
>>   	  thousands of CPUs.  It also scales down nicely to
>>   	  smaller systems.
>>   
>> +config ARCH_HAVE_RCU_PREEMPT_DEEPTH
>> +	def_bool n
>> +
>>   config PREEMPT_RCU
>>   	bool
>>   	default y if PREEMPTION
>> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
>> index dcb2124203cf..f919881832d4 100644
>> --- a/kernel/rcu/tree_exp.h
>> +++ b/kernel/rcu/tree_exp.h
>> @@ -588,6 +588,7 @@ static void wait_rcu_exp_gp(struct work_struct *wp)
>>   }
>>   
>>   #ifdef CONFIG_PREEMPT_RCU
>> +static inline void set_rcu_preempt_need_special(void);
>>   
>>   /*
>>    * Remote handler for smp_call_function_single().  If there is an
>> @@ -637,6 +638,7 @@ static void rcu_exp_handler(void *unused)
>>   		if (rnp->expmask & rdp->grpmask) {
>>   			rdp->exp_deferred_qs = true;
>>   			t->rcu_read_unlock_special.b.exp_hint = true;
>> +			set_rcu_preempt_need_special();
>>   		}
>>   		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>>   		return;
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index e16c3867d2ff..e6774a7ab16b 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -82,7 +82,7 @@ static void __init rcu_bootup_announce_oddness(void)
>>   #ifdef CONFIG_PREEMPT_RCU
>>   
>>   static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
>> -static void rcu_read_unlock_special(struct task_struct *t);
>> +void rcu_read_unlock_special(void);
>>   
>>   /*
>>    * Tell them what RCU they are running.
>> @@ -298,6 +298,7 @@ void rcu_note_context_switch(bool preempt)
>>   		t->rcu_read_unlock_special.b.need_qs = false;
>>   		t->rcu_read_unlock_special.b.blocked = true;
>>   		t->rcu_blocked_node = rnp;
>> +		set_rcu_preempt_need_special();
>>   
>>   		/*
>>   		 * Verify the CPU's sanity, trace the preemption, and
>> @@ -345,6 +346,7 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
>>   /* Bias and limit values for ->rcu_read_lock_nesting. */
>>   #define RCU_NEST_PMAX (INT_MAX / 2)
>>   
>> +#ifndef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH
>>   static inline void rcu_preempt_depth_inc(void)
>>   {
>>   	current->rcu_read_lock_nesting++;
>> @@ -352,7 +354,12 @@ static inline void rcu_preempt_depth_inc(void)
>>   
>>   static inline bool rcu_preempt_depth_dec_and_test(void)
>>   {
>> -	return --current->rcu_read_lock_nesting == 0;
>> +	if (--current->rcu_read_lock_nesting == 0) {
>> +		/* check speical after dec ->rcu_read_lock_nesting */
>> +		barrier();
>> +		return READ_ONCE(current->rcu_read_unlock_special.s);
>> +	}
>> +	return 0;
>>   }
>>   
>>   static inline void rcu_preempt_depth_set(int val)
>> @@ -360,6 +367,12 @@ static inline void rcu_preempt_depth_set(int val)
>>   	current->rcu_read_lock_nesting = val;
>>   }
>>   
>> +static inline void clear_rcu_preempt_need_special(void) {}
>> +static inline void set_rcu_preempt_need_special(void) {}
>> +
>> +#endif /* #ifndef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH */
>> +
>> +#if !defined(CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH) || defined (CONFIG_PROVE_LOCKING)
>>   /*
>>    * Preemptible RCU implementation for rcu_read_lock().
>>    * Just increment ->rcu_read_lock_nesting, shared state will be updated
>> @@ -383,18 +396,16 @@ EXPORT_SYMBOL_GPL(__rcu_read_lock);
>>    */
>>   void __rcu_read_unlock(void)
>>   {
>> -	struct task_struct *t = current;
>> -
>>   	if (rcu_preempt_depth_dec_and_test()) {
>> -		barrier();  /* critical section before exit code. */
>> -		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
>> -			rcu_read_unlock_special(t);
>> +		rcu_read_unlock_special();
>>   	}
>>   	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
>> -		WARN_ON_ONCE(rcu_preempt_depth() < 0);
>> +		WARN_ON_ONCE(rcu_preempt_depth() < 0 ||
>> +			     rcu_preempt_depth() > RCU_NEST_PMAX);
>>   	}
>>   }
>>   EXPORT_SYMBOL_GPL(__rcu_read_unlock);
>> +#endif /* #if !defined(CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH) || defined (CONFIG_PROVE_LOCKING) */
>>   
>>   /*
>>    * Advance a ->blkd_tasks-list pointer to the next entry, instead
>> @@ -449,6 +460,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>>   		return;
>>   	}
>>   	t->rcu_read_unlock_special.s = 0;
>> +	clear_rcu_preempt_need_special();
>>   	if (special.b.need_qs)
>>   		rcu_qs();
>>   
>> @@ -579,8 +591,9 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
>>    * notify RCU core processing or task having blocked during the RCU
>>    * read-side critical section.
>>    */
>> -static void rcu_read_unlock_special(struct task_struct *t)
>> +void rcu_read_unlock_special(void)
>>   {
>> +	struct task_struct *t = current;
>>   	unsigned long flags;
>>   	bool preempt_bh_were_disabled =
>>   			!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
>> @@ -631,6 +644,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
>>   	}
>>   	rcu_preempt_deferred_qs_irqrestore(t, flags);
>>   }
>> +EXPORT_SYMBOL_GPL(rcu_read_unlock_special);
>>   
>>   /*
>>    * Check that the list of blocked tasks for the newly completed grace
>> @@ -694,8 +708,10 @@ static void rcu_flavor_sched_clock_irq(int user)
>>   	    __this_cpu_read(rcu_data.core_needs_qs) &&
>>   	    __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
>>   	    !t->rcu_read_unlock_special.b.need_qs &&
>> -	    time_after(jiffies, rcu_state.gp_start + HZ))
>> +	    time_after(jiffies, rcu_state.gp_start + HZ)) {
>>   		t->rcu_read_unlock_special.b.need_qs = true;
>> +		set_rcu_preempt_need_special();
>> +	}
>>   }
>>   
>>   /*
>> @@ -714,6 +730,7 @@ void exit_rcu(void)
>>   		rcu_preempt_depth_set(1);
>>   		barrier();
>>   		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
>> +		set_rcu_preempt_need_special();
>>   	} else if (unlikely(rcu_preempt_depth())) {
>>   		rcu_preempt_depth_set(1);
>>   	} else {
>> -- 
>> 2.20.1
>>