[PATCH] x86_64: Switch to the interrupt stack when running a softirq in local_bh_enable()

This avoids some potential stack overflows with very deep softirq callchains.
i386 does this too.

TOADD CFI annotation

Signed-off-by: Andi Kleen <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86_64/kernel/entry.S

···
 ENTRY(call_debug)
        zeroentry do_call_debug
 
+ENTRY(call_softirq)
+	movq %gs:pda_irqstackptr,%rax
+	pushq %r15
+	movq %rsp,%r15
+	incl %gs:pda_irqcount
+	cmove %rax,%rsp
+	call __do_softirq
+	movq %r15,%rsp
+	decl %gs:pda_irqcount
+	popq %r15
+	ret
+

arch/x86_64/kernel/irq.c

···
 	local_irq_disable();
 }
 #endif
+
+extern void call_softirq(void);
+
+asmlinkage void do_softirq(void)
+{
+ 	__u32 pending;
+ 	unsigned long flags;
+
+ 	if (in_interrupt())
+ 		return;
+
+ 	local_irq_save(flags);
+ 	pending = local_softirq_pending();
+ 	/* Switch to interrupt stack */
+ 	if (pending)
+		call_softirq();
+ 	local_irq_restore(flags);
+}
+EXPORT_SYMBOL(do_softirq);

include/asm-x86_64/irq.h

···
 extern void fixup_irqs(cpumask_t map);
 #endif
 
+#define __ARCH_HAS_DO_SOFTIRQ 1
+
 #endif /* _ASM_IRQ_H */

kernel/softirq.c

···
 	/* Reset the pending bitmask before enabling irqs */
 	local_softirq_pending() = 0;
 
-	local_irq_enable();
+	//local_irq_enable();
 
 	h = softirq_vec;
 
···
 		pending >>= 1;
 	} while (pending);
 
-	local_irq_disable();
+	//local_irq_disable();
 
 	pending = local_softirq_pending();
 	if (pending && --max_restart)