arm64: remove irq_count and do_softirq_own_stack()

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 thread_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@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
[will: use get_thread_info macro]
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 10 insertions, 9 deletions

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 0667fb7d8bb1..c0db321db7e1 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 with the current thread_info, 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?
+        and     x25, x19, #~(THREAD_SIZE - 1)
+        cmp     x25, tsk
+        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
 
         /*
@@ -405,10 +406,10 @@ el1_irq:
         bl      trace_hardirqs_off
 #endif
 
+        get_thread_info tsk
         irq_handler
 
 #ifdef CONFIG_PREEMPT
-        get_thread_info tsk
         ldr     w24, [tsk, #TI_PREEMPT]         // get preempt count
         cbnz    w24, 1f                         // preempt count != 0
         ldr     x0, [tsk, #TI_FLAGS]            // get flags