From: Thomas Gleixner <t...@linutronix.de> The recursion protection for hard interrupt stacks is an unsigned int per CPU variable initialized to -1 named __irq_count.
The irq stack switching is only done when the variable is -1, which creates worse code than just checking for 0. When the stack switching happens it uses this_cpu_add/sub(1), but there is no reason to do so. It simply can use straight writes. This is a historical leftover from the low level ASM code which used inc and jz to make a decision. Rename it to hardirq_stack_inuse, make it a bool and use plain stores. Signed-off-by: Thomas Gleixner <t...@linutronix.de> Reviewed-by: Kees Cook <keesc...@chromium.org> --- arch/x86/include/asm/irq_stack.h | 14 +++++++------- arch/x86/include/asm/processor.h | 2 +- arch/x86/kernel/cpu/common.c | 2 +- arch/x86/kernel/process_64.c | 2 +- 4 files changed, 10 insertions(+), 10 deletions(-) --- a/arch/x86/include/asm/irq_stack.h +++ b/arch/x86/include/asm/irq_stack.h @@ -9,7 +9,7 @@ #ifdef CONFIG_X86_64 static __always_inline bool irqstack_active(void) { - return __this_cpu_read(irq_count) != -1; + return __this_cpu_read(hardirq_stack_inuse); } void asm_call_on_stack(void *sp, void (*func)(void), void *arg); @@ -22,9 +22,9 @@ static __always_inline void __run_on_irq { void *tos = __this_cpu_read(hardirq_stack_ptr); - __this_cpu_add(irq_count, 1); + __this_cpu_write(hardirq_stack_inuse, true); asm_call_on_stack(tos - 8, func, NULL); - __this_cpu_sub(irq_count, 1); + __this_cpu_write(hardirq_stack_inuse, false); } static __always_inline void @@ -33,9 +33,9 @@ static __always_inline void { void *tos = __this_cpu_read(hardirq_stack_ptr); - __this_cpu_add(irq_count, 1); + __this_cpu_write(hardirq_stack_inuse, true); asm_call_sysvec_on_stack(tos - 8, func, regs); - __this_cpu_sub(irq_count, 1); + __this_cpu_write(hardirq_stack_inuse, false); } static __always_inline void @@ -44,9 +44,9 @@ static __always_inline void { void *tos = __this_cpu_read(hardirq_stack_ptr); - __this_cpu_add(irq_count, 1); + __this_cpu_write(hardirq_stack_inuse, true); asm_call_irq_on_stack(tos - 8, func, desc); - __this_cpu_sub(irq_count, 1); + __this_cpu_write(hardirq_stack_inuse, false); } #else /* CONFIG_X86_64 */ --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -454,7 +454,7 @@ static inline unsigned long cpu_kernelmo return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu); } -DECLARE_PER_CPU(unsigned int, irq_count); +DECLARE_PER_CPU(bool, hardirq_stack_inuse); extern asmlinkage void ignore_sysret(void); /* Save actual FS/GS selectors and bases to current->thread */ --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1740,7 +1740,7 @@ DEFINE_PER_CPU(struct task_struct *, cur EXPORT_PER_CPU_SYMBOL(current_task); DEFINE_PER_CPU(struct irq_stack *, hardirq_stack_ptr); -DEFINE_PER_CPU(unsigned int, irq_count) __visible = -1; +DEFINE_PER_CPU(bool, hardirq_stack_inuse); DEFINE_PER_CPU(int, __preempt_count) = INIT_PREEMPT_COUNT; EXPORT_PER_CPU_SYMBOL(__preempt_count); --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -539,7 +539,7 @@ void compat_start_thread(struct pt_regs int cpu = smp_processor_id(); WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ENTRY) && - this_cpu_read(irq_count) != -1); + this_cpu_read(hardirq_stack_inuse)); if (!test_thread_flag(TIF_NEED_FPU_LOAD)) switch_fpu_prepare(prev_fpu, cpu);