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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Subject: [RFC PATCH 8/8] powerpc/irq: drop softirq stack
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Date:   Mon, 23 Dec 2019 15:26:19 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

There are two IRQ stacks: softirq_ctx and hardirq_ctx

do_softirq_own_stack() switches stack to softirq_ctx
do_IRQ() switches stack to hardirq_ctx

However, when soft and hard IRQs are nested, only one of the two
stacks is used:
- When on softirq stack, do_IRQ() doesn't switch to hardirq stack.
- irq_exit() runs softirqs on hardirq stack.

There is no added value in having two IRQ stacks as only one is
used when hard and soft irqs are nested. Remove softirq_ctx and
use hardirq_ctx for both hard and soft IRQs.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
---
 arch/powerpc/include/asm/irq.h | 1 -
 arch/powerpc/kernel/irq.c      | 8 +++-----
 arch/powerpc/kernel/process.c  | 4 ----
 arch/powerpc/kernel/setup_32.c | 4 +---
 arch/powerpc/kernel/setup_64.c | 4 +---
 5 files changed, 5 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/include/asm/irq.h b/arch/powerpc/include/asm/irq.h
index e4a92f0b4ad4..7cb2c76aa3ed 100644
--- a/arch/powerpc/include/asm/irq.h
+++ b/arch/powerpc/include/asm/irq.h
@@ -54,7 +54,6 @@ extern void *mcheckirq_ctx[NR_CPUS];
  * Per-cpu stacks for handling hard and soft interrupts.
  */
 extern void *hardirq_ctx[NR_CPUS];
-extern void *softirq_ctx[NR_CPUS];
 
 #ifdef CONFIG_PPC64
 void call_do_softirq(void *sp);
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index a1122ef4a16c..3af0d1897354 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -680,15 +680,14 @@ void __do_irq(struct pt_regs *regs)
 
 void do_IRQ(struct pt_regs *regs)
 {
-	void *cursp, *irqsp, *sirqsp;
+	void *cursp, *irqsp;
 
 	/* Switch to the irq stack to handle this */
 	cursp = (void *)(stack_pointer() & ~(THREAD_SIZE - 1));
 	irqsp = hardirq_ctx[raw_smp_processor_id()];
-	sirqsp = softirq_ctx[raw_smp_processor_id()];
 
 	/* Already there ? Otherwise switch stack and call */
-	if (unlikely(cursp == irqsp || cursp == sirqsp))
+	if (unlikely(cursp == irqsp))
 		__do_irq(regs);
 	else
 		call_do_irq(regs, irqsp);
@@ -706,12 +705,11 @@ void    *dbgirq_ctx[NR_CPUS] __read_mostly;
 void *mcheckirq_ctx[NR_CPUS] __read_mostly;
 #endif
 
-void *softirq_ctx[NR_CPUS] __read_mostly;
 void *hardirq_ctx[NR_CPUS] __read_mostly;
 
 void do_softirq_own_stack(void)
 {
-	call_do_softirq(softirq_ctx[smp_processor_id()]);
+	call_do_softirq(hardirq_ctx[smp_processor_id()]);
 }
 
 irq_hw_number_t virq_to_hw(unsigned int virq)
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 49d0ebf28ab9..be3e64cf28b4 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1963,10 +1963,6 @@ static inline int valid_irq_stack(unsigned long sp, struct task_struct *p,
 	if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
 		return 1;
 
-	stack_page = (unsigned long)softirq_ctx[cpu];
-	if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
-		return 1;
-
 	return 0;
 }
 
diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
index dcffe927f5b9..8752aae06177 100644
--- a/arch/powerpc/kernel/setup_32.c
+++ b/arch/powerpc/kernel/setup_32.c
@@ -155,10 +155,8 @@ void __init irqstack_early_init(void)
 
 	/* interrupt stacks must be in lowmem, we get that for free on ppc32
 	 * as the memblock is limited to lowmem by default */
-	for_each_possible_cpu(i) {
-		softirq_ctx[i] = alloc_stack();
+	for_each_possible_cpu(i)
 		hardirq_ctx[i] = alloc_stack();
-	}
 }
 
 #if defined(CONFIG_BOOKE) || defined(CONFIG_40x)
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index 6104917a282d..96ee7627eda6 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -652,10 +652,8 @@ void __init irqstack_early_init(void)
 	 * cannot afford to take SLB misses on them. They are not
 	 * accessed in realmode.
 	 */
-	for_each_possible_cpu(i) {
-		softirq_ctx[i] = alloc_stack(limit, i);
+	for_each_possible_cpu(i)
 		hardirq_ctx[i] = alloc_stack(limit, i);
-	}
 }
 
 #ifdef CONFIG_PPC_BOOK3E
-- 
2.13.3