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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>
 <4680b6f5-02d2-1d76-0b67-a5d3d9b36200@linux.alibaba.com>
 <20191118145953.GK2889@paulmck-ThinkPad-P72>
From:   Lai Jiangshan <laijs@linux.alibaba.com>
Message-ID: <3dd0fa56-68b9-c179-95db-f110e47f53b7@linux.alibaba.com>
Date:   Tue, 19 Nov 2019 09:59:03 +0800
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On 2019/11/18 10:59 下午, Paul E. McKenney wrote:
> On Mon, Nov 18, 2019 at 10:02:50AM +0800, Lai Jiangshan wrote:
>>
>>
>> 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/.
> 
> True enough.
> 
> But doesn't the per-CPU code require per-architecture changes to copy
> to and from the per-CPU variable?  If that code simpler and smaller than
> the thread_info access code, I will be -very- surprised.
> 

The per-CPU code is not simpler. And my code touch X86 ONLY so that
it requires an additional CONFIG and some more "#if" in rcu code
which adds little more complicity.

As far as I understand, the rcu_read_lock_nesting can only be possible
put in per CPU in X86, not possible in other ARCHs.

Putting the rcu_read_lock_nesting in struct thread_info and putting
the inlining functions in include/asm-generic/ are OK for me. It will
be a good-shaped framework and reduce function calls, and still allow
x86 has its own implementation.

The framework will only be more appealing when the x86 percpu
rcu_read_lock_nesting is implemented. This series implements
the x86 percpu rcu_read_lock_nesting first and avoid touch too much
files. The framework can be implemented later

Thanks
Lai


> 
> 							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
>>>>