powerpc/irq: Allow softirq to hardirq stack transition

Allow a transition from the softirq stack to the hardirq stack when
handling a hardirq. Doing so means a hardirq received while deep in
softirq processing is less likely to cause a stack overflow of the
softirq stack.

Previously it wasn't safe to do so because irq_exit() (which initiates
softirq processing) was called on the hardirq stack.

That was changed in commit 1b1b6a6 ("powerpc: handle irq_enter/
irq_exit in interrupt handler wrappers") and 1346d00 ("powerpc:
Don't select HAVE_IRQ_EXIT_ON_IRQ_STACK").

The allowed transitions are now:
 - process stack -> hardirq stack
 - process stack -> softirq stack
 - process stack -> softirq stack -> hardirq stack

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231130125045.3080961-1-mpe@ellerman.id.au

Author: mpe

arch/powerpc/kernel/irq.c

@@ -284,15 +284,14 @@ static __always_inline void call_do_irq(struct pt_regs *regs, void *sp)
 void __do_IRQ(struct pt_regs *regs)
 {
 	struct pt_regs *old_regs = set_irq_regs(regs);
-	void *cursp, *irqsp, *sirqsp;
+	void *cursp, *irqsp;
 
 	/* Switch to the irq stack to handle this */
 	cursp = (void *)(current_stack_pointer & ~(THREAD_SIZE - 1));
 	irqsp = hardirq_ctx[raw_smp_processor_id()];
-	sirqsp = softirq_ctx[raw_smp_processor_id()];
 
 	/* Already there ? If not switch stack and call */
-	if (unlikely(cursp == irqsp || cursp == sirqsp))
+	if (unlikely(cursp == irqsp))
 		__do_irq(regs, current_stack_pointer);
 	else
 		call_do_irq(regs, irqsp);