[PATCH v8 9/4] arm64: remove irq_count and do_softirq_own_stack()

[James Morse]

sysrq_handle_reboot() re-enables interrupts while on the irq stack. The
irq_stack implementation wrongly assumed this would only ever happen
via the softirq path, allowing it to update irq_count late, in
do_softirq_own_stack().

This means if an irq occurs in sysrq_handle_reboot(), during
emergency_restart() the stack will be corrupted, as irq_count wasn't
updated.

Lose the optimisation, and instead of moving the adding/subtracting of
irq_count into irq_stack_entry/irq_stack_exit, remove it, and compare
sp_el0 (struct task_info) with sp & ~(THREAD_SIZE - 1). This tells us if
we are on a task stack, if so, we can safely switch to the irq stack.
Finally, remove do_softirq_own_stack(), we don't need it anymore.

Reported-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: James Morse <james.morse at arm.com>
---
 arch/arm64/include/asm/irq.h |  2 --
 arch/arm64/kernel/entry.S    | 17 +++++++++--------
 arch/arm64/kernel/irq.c      | 38 +-------------------------------------
 3 files changed, 10 insertions(+), 47 deletions(-)

diff --git a/arch/arm64/include/asm/irq.h b/arch/arm64/include/asm/irq.h
index 3bece4379bd9..b77197d941fc 100644
--- a/arch/arm64/include/asm/irq.h
+++ b/arch/arm64/include/asm/irq.h
@@ -11,8 +11,6 @@
 #include <asm-generic/irq.h>
 #include <asm/thread_info.h>
 
-#define __ARCH_HAS_DO_SOFTIRQ
-
 struct pt_regs;
 
 DECLARE_PER_CPU(unsigned long [IRQ_STACK_SIZE/sizeof(long)], irq_stack);
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 0667fb7d8bb1..6745a9041f99 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -181,19 +181,20 @@ alternative_endif
 	.macro	irq_stack_entry
 	mov	x19, sp			// preserve the original sp
 
-	this_cpu_ptr irq_stack, x25, x26
-
 	/*
-	 * Check the lowest address on irq_stack for the irq_count value,
-	 * incremented by do_softirq_own_stack if we have re-enabled irqs
-	 * while on the irq_stack.
+	 * Compare sp and sp_el0, if the top ~(THREAD_SIZE - 1) bits match,
+	 * we are on a task stack, and should switch to the irq stack.
 	 */
-	ldr	x26, [x25]
-	cbnz	x26, 9998f		// recursive use?
+	mrs	x26, sp_el0		// already masked
+	and	x25, x19, #~(THREAD_SIZE - 1)
+	cmp	x25, x26
+	b.ne	9998f
 
-	/* switch to the irq stack */
+	this_cpu_ptr irq_stack, x25, x26
 	mov	x26, #IRQ_STACK_START_SP
 	add	x26, x25, x26
+
+	/* switch to the irq stack */
 	mov	sp, x26
 
 	/*
diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
index ff7ebb710e51..2386b26c0712 100644
--- a/arch/arm64/kernel/irq.c
+++ b/arch/arm64/kernel/irq.c
@@ -25,24 +25,14 @@
 #include <linux/irq.h>
 #include <linux/smp.h>
 #include <linux/init.h>
-#include <linux/interrupt.h>
 #include <linux/irqchip.h>
 #include <linux/seq_file.h>
 
 unsigned long irq_err_count;
 
-/*
- * irq stack only needs to be 16 byte aligned - not IRQ_STACK_SIZE aligned.
- * irq_stack[0] is used as irq_count, a non-zero value indicates the stack
- * is in use, and el?_irq() shouldn't switch to it. This is used to detect
- * recursive use of the irq_stack, it is lazily updated by
- * do_softirq_own_stack(), which is called on the irq_stack, before
- * re-enabling interrupts to process softirqs.
- */
+/* irq stack only needs to be 16 byte aligned - not IRQ_STACK_SIZE aligned. */
 DEFINE_PER_CPU(unsigned long [IRQ_STACK_SIZE/sizeof(long)], irq_stack) __aligned(16);
 
-#define IRQ_COUNT()	(*per_cpu(irq_stack, smp_processor_id()))
-
 int arch_show_interrupts(struct seq_file *p, int prec)
 {
 	show_ipi_list(p, prec);
@@ -66,29 +56,3 @@ void __init init_IRQ(void)
 	if (!handle_arch_irq)
 		panic("No interrupt controller found.");
 }
-
-/*
- * do_softirq_own_stack() is called from irq_exit() before __do_softirq()
- * re-enables interrupts, at which point we may re-enter el?_irq(). We
- * increase irq_count here so that el1_irq() knows that it is already on the
- * irq stack.
- *
- * Called with interrupts disabled, so we don't worry about moving cpu, or
- * being interrupted while modifying irq_count.
- *
- * This function doesn't actually switch stack.
- */
-void do_softirq_own_stack(void)
-{
-	int cpu = smp_processor_id();
-
-	WARN_ON_ONCE(!irqs_disabled());
-
-	if (on_irq_stack(current_stack_pointer, cpu)) {
-		IRQ_COUNT()++;
-		__do_softirq();
-		IRQ_COUNT()--;
-	} else {
-		__do_softirq();
-	}
-}
-- 
2.6.2