linux-kernel - [PATCH 8/8] x86: remove 64-byte gap at end of irq stack

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Date:   Thu, 18 Aug 2016 10:59:08 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: [PATCH 8/8] x86: remove 64-byte gap at end of irq stack

There has been a 64-byte gap at the end of the irq stack for at least 12
years.  It predates git history, and I can't find any good reason for
it.  Remove it.  What's the worst that could happen?

Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/kernel/cpu/common.c   | 2 +-
 arch/x86/kernel/dumpstack_64.c | 9 +++------
 arch/x86/kernel/setup_percpu.c | 2 +-
 3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index d3b91be..ce7a4c1 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1286,7 +1286,7 @@ DEFINE_PER_CPU(struct task_struct *, current_task) ____cacheline_aligned =
 EXPORT_PER_CPU_SYMBOL(current_task);
 
 DEFINE_PER_CPU(char *, irq_stack_ptr) =
-	init_per_cpu_var(irq_stack_union.irq_stack) + IRQ_STACK_SIZE - 64;
+	init_per_cpu_var(irq_stack_union.irq_stack) + IRQ_STACK_SIZE;
 
 DEFINE_PER_CPU(unsigned int, irq_count) __visible = -1;
 
diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index daf9f63..066eb5c7 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -103,9 +103,6 @@ in_irq_stack(unsigned long *stack, unsigned long *irq_stack,
 	return (stack >= irq_stack && stack < irq_stack_end);
 }
 
-static const unsigned long irq_stack_size =
-	(IRQ_STACK_SIZE - 64) / sizeof(unsigned long);
-
 enum stack_type {
 	STACK_IS_UNKNOWN,
 	STACK_IS_NORMAL,
@@ -133,7 +130,7 @@ analyze_stack(int cpu, struct task_struct *task, unsigned long *stack,
 		return STACK_IS_NORMAL;
 
 	*stack_end = irq_stack;
-	irq_stack = irq_stack - irq_stack_size;
+	irq_stack -= (IRQ_STACK_SIZE / sizeof(long));
 
 	if (in_irq_stack(stack, irq_stack, *stack_end))
 		return STACK_IS_IRQ;
@@ -256,8 +253,8 @@ show_stack_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	preempt_disable();
 	cpu = smp_processor_id();
 
-	irq_stack_end	= (unsigned long *)(per_cpu(irq_stack_ptr, cpu));
-	irq_stack	= (unsigned long *)(per_cpu(irq_stack_ptr, cpu) - IRQ_STACK_SIZE);
+	irq_stack_end = (unsigned long *)(per_cpu(irq_stack_ptr, cpu));
+	irq_stack     = irq_stack_end - (IRQ_STACK_SIZE / sizeof(long));
 
 	/*
 	 * Debugging aid: "show_stack(NULL, NULL);" prints the
diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index 1d5c794..2bbd27f 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -246,7 +246,7 @@ void __init setup_per_cpu_areas(void)
 #ifdef CONFIG_X86_64
 		per_cpu(irq_stack_ptr, cpu) =
 			per_cpu(irq_stack_union.irq_stack, cpu) +
-			IRQ_STACK_SIZE - 64;
+			IRQ_STACK_SIZE;
 #endif
 #ifdef CONFIG_NUMA
 		per_cpu(x86_cpu_to_node_map, cpu) =
-- 
2.7.4