ARM: run softirqs on the per-CPU IRQ stack

Now that we have enabled IRQ stacks, any softIRQs that are handled over
the back of a hard IRQ will run from the IRQ stack as well. However, any
synchronous softirq processing that happens when re-enabling softIRQs
from task context will still execute on that task's stack.

Since any call to local_bh_enable() at any level in the task's call
stack may trigger a softIRQ processing run, which could potentially
cause a task stack overflow if the combined stack footprints exceed the
stack's size, let's run these synchronous invocations of do_softirq() on
the IRQ stack as well.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Keith Packard <keithpac@amazon.com>
Tested-by: Marc Zyngier <maz@kernel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M

arch/arm/Kconfig

@@ -1169,6 +1169,8 @@ config CURRENT_POINTER_IN_TPIDRURO
 config IRQSTACKS
 	def_bool y
 	depends on GENERIC_IRQ_MULTI_HANDLER && THREAD_INFO_IN_TASK
+	select HAVE_IRQ_EXIT_ON_IRQ_STACK
+	select HAVE_SOFTIRQ_ON_OWN_STACK
 
 config ARM_CPU_TOPOLOGY
 	bool "Support cpu topology definition"

arch/arm/kernel/irq.c

@@ -36,11 +36,14 @@
 #include <asm/hardware/cache-l2x0.h>
 #include <asm/hardware/cache-uniphier.h>
 #include <asm/outercache.h>
+#include <asm/softirq_stack.h>
 #include <asm/exception.h>
 #include <asm/mach/arch.h>
 #include <asm/mach/irq.h>
 #include <asm/mach/time.h>
 
+#include "reboot.h"
+
 unsigned long irq_err_count;
 
 #ifdef CONFIG_IRQSTACKS
@@ -60,6 +63,17 @@ static void __init init_irq_stacks(void)
 	}
 }
 
+static void ____do_softirq(void *arg)
+{
+	__do_softirq();
+}
+
+void do_softirq_own_stack(void)
+{
+	call_with_stack(____do_softirq, NULL,
+			__this_cpu_read(irq_stack_ptr));
+}
+
 #endif
 
 int arch_show_interrupts(struct seq_file *p, int prec)