On Tue, 27 Nov 2018 at 20:44, Will Deacon <will.dea...@arm.com> wrote: > > The asm-generic/preempt.h implementation doesn't make use of the > PREEMPT_NEED_RESCHED flag, since this can interact badly with load/store > architectures which rely on the preempt_count word being unchanged across > an interrupt. > > However, since we're a 64-bit architecture and the preempt count is > only 32 bits wide, we can simply pack it next to the resched flag and > load the whole thing in one go, so that a dec-and-test operation doesn't > need to load twice. >
Since the actual preempt count is a lot narrower than 32 bits, x86 just uses bit 31. So what is the reason for using two different words? > Signed-off-by: Will Deacon <will.dea...@arm.com> > --- > arch/arm64/include/asm/Kbuild | 1 - > arch/arm64/include/asm/preempt.h | 78 > ++++++++++++++++++++++++++++++++++++ > arch/arm64/include/asm/thread_info.h | 13 +++++- > 3 files changed, 90 insertions(+), 2 deletions(-) > create mode 100644 arch/arm64/include/asm/preempt.h > > diff --git a/arch/arm64/include/asm/Kbuild b/arch/arm64/include/asm/Kbuild > index 6cd5d77b6b44..33498f900390 100644 > --- a/arch/arm64/include/asm/Kbuild > +++ b/arch/arm64/include/asm/Kbuild > @@ -14,7 +14,6 @@ generic-y += local64.h > generic-y += mcs_spinlock.h > generic-y += mm-arch-hooks.h > generic-y += msi.h > -generic-y += preempt.h > generic-y += qrwlock.h > generic-y += qspinlock.h > generic-y += rwsem.h > diff --git a/arch/arm64/include/asm/preempt.h > b/arch/arm64/include/asm/preempt.h > new file mode 100644 > index 000000000000..832227d5ebc0 > --- /dev/null > +++ b/arch/arm64/include/asm/preempt.h > @@ -0,0 +1,78 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +#ifndef __ASM_PREEMPT_H > +#define __ASM_PREEMPT_H > + > +#include <linux/thread_info.h> > + > +#define PREEMPT_NEED_RESCHED BIT(32) > +#define PREEMPT_ENABLED (PREEMPT_NEED_RESCHED) > + > +static inline int preempt_count(void) > +{ > + return READ_ONCE(current_thread_info()->preempt.count); > +} > + > +static inline void preempt_count_set(u64 pc) > +{ > + /* Preserve existing value of PREEMPT_NEED_RESCHED */ > + WRITE_ONCE(current_thread_info()->preempt.count, pc); > +} > + > +#define init_task_preempt_count(p) do { \ > + task_thread_info(p)->preempt_count = FORK_PREEMPT_COUNT; \ > +} while (0) > + > +#define init_idle_preempt_count(p, cpu) do { \ > + task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \ > +} while (0) > + > +static inline void set_preempt_need_resched(void) > +{ > + current_thread_info()->preempt.need_resched = 0; > +} > + > +static inline void clear_preempt_need_resched(void) > +{ > + current_thread_info()->preempt.need_resched = 1; > +} > + > +static inline bool test_preempt_need_resched(void) > +{ > + return !current_thread_info()->preempt.need_resched; > +} > + > +static inline void __preempt_count_add(int val) > +{ > + u32 pc = READ_ONCE(current_thread_info()->preempt.count); > + pc += val; > + WRITE_ONCE(current_thread_info()->preempt.count, pc); > +} > + > +static inline void __preempt_count_sub(int val) > +{ > + u32 pc = READ_ONCE(current_thread_info()->preempt.count); > + pc -= val; > + WRITE_ONCE(current_thread_info()->preempt.count, pc); > +} > + > +static inline bool __preempt_count_dec_and_test(void) > +{ > + u64 pc = READ_ONCE(current_thread_info()->preempt_count); > + WRITE_ONCE(current_thread_info()->preempt.count, --pc); > + return !pc; > +} > + > +static inline bool should_resched(int preempt_offset) > +{ > + u64 pc = READ_ONCE(current_thread_info()->preempt_count); > + return pc == preempt_offset; > +} > + > +#ifdef CONFIG_PREEMPT > +void preempt_schedule(void); > +#define __preempt_schedule() preempt_schedule() > +void preempt_schedule_notrace(void); > +#define __preempt_schedule_notrace() preempt_schedule_notrace() > +#endif /* CONFIG_PREEMPT */ > + > +#endif /* __ASM_PREEMPT_H */ > diff --git a/arch/arm64/include/asm/thread_info.h > b/arch/arm64/include/asm/thread_info.h > index cb2c10a8f0a8..bbca68b54732 100644 > --- a/arch/arm64/include/asm/thread_info.h > +++ b/arch/arm64/include/asm/thread_info.h > @@ -42,7 +42,18 @@ struct thread_info { > #ifdef CONFIG_ARM64_SW_TTBR0_PAN > u64 ttbr0; /* saved TTBR0_EL1 */ > #endif > - int preempt_count; /* 0 => preemptable, <0 => > bug */ > + union { > + u64 preempt_count; /* 0 => preemptible, <0 => > bug */ > + struct { > +#ifdef CONFIG_CPU_BIG_ENDIAN > + u32 need_resched; > + u32 count; > +#else > + u32 count; > + u32 need_resched; > +#endif > + } preempt; > + }; > }; > > #define thread_saved_pc(tsk) \ > -- > 2.1.4 >