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Message-ID: <20191118145953.GK2889@paulmck-ThinkPad-P72>
Date: Mon, 18 Nov 2019 06:59:53 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Lai Jiangshan <laijs@...ux.alibaba.com>
Cc: linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>,
Andi Kleen <ak@...ux.intel.com>,
Andy Lutomirski <luto@...nel.org>,
Fenghua Yu <fenghua.yu@...el.com>,
Kees Cook <keescook@...omium.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Dave Hansen <dave.hansen@...el.com>,
Babu Moger <Babu.Moger@....com>,
Rik van Riel <riel@...riel.com>,
"Chang S. Bae" <chang.seok.bae@...el.com>,
Jann Horn <jannh@...gle.com>,
David Windsor <dwindsor@...il.com>,
Elena Reshetova <elena.reshetova@...el.com>,
Yuyang Du <duyuyang@...il.com>,
Anshuman Khandual <anshuman.khandual@....com>,
Richard Guy Briggs <rgb@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Christian Brauner <christian.brauner@...ntu.com>,
Michal Hocko <mhocko@...e.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Al Viro <viro@...iv.linux.org.uk>,
"Dmitry V. Levin" <ldv@...linux.org>, rcu@...r.kernel.org
Subject: Re: [PATCH V2 7/7] x86,rcu: use percpu rcu_preempt_depth
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@...radead.org>
> > > Signed-off-by: Lai Jiangshan <laijs@...ux.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.
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
> > >
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