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Subject[patch 02/12] x86/irq: Sanitize irq stack tracking
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 <tglx@linutronix.de>
---
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);
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