[PATCH RFC] riscv: disable local interrupts and stop other CPUs before restart

[Troy Mitchell]

Currently, the RISC-V implementation of machine_restart() directly calls
do_kernel_restart() without disabling local interrupts or stopping other
CPUs. This missing architectural setup causes fatal issues for systems
that rely on external peripherals (e.g., I2C PMICs) to execute the system
restart when CONFIG_PREEMPT_RCU is enabled.

When a restart handler relies on the I2C subsystem, the I2C core checks
i2c_in_atomic_xfer_mode() to decide whether to use the sleepable xfer
or the polling atomic_xfer. This check evaluates to true if
(!preemptible() || irqs_disabled()).

During do_kernel_restart(), the restart handlers are invoked via
atomic_notifier_call_chain(), which holds an RCU read lock.
The behavior diverges based on the preemption model:
1. Under CONFIG_PREEMPT_VOLUNTARY or CONFIG_PREEMPT_NONE, rcu_read_lock()
   implicitly disables preemption. preemptible() evaluates to false, and
   the I2C core correctly routes to the atomic, polling transfer path.
2. Under CONFIG_PREEMPT_RCU, rcu_read_lock() does NOT disable preemption.
   Since machine_restart() left local interrupts enabled, irqs_disabled()
   is false, and preempt_count is 0. Consequently, preemptible() evaluates
   to true.

As a result, the I2C core falsely assumes a sleepable context and routes
the transfer to the standard master_xfer path. This inevitably triggers a
schedule() call while holding the RCU read lock, resulting in a fatal splat:
"Voluntary context switch within RCU read-side critical section!" and
a system hang.

Align RISC-V with other major architectures (e.g., ARM64) by adding
local_irq_disable() and smp_send_stop() to machine_restart().
- local_irq_disable() guarantees a strict atomic context, forcing sub-
  systems like I2C to always fall back to polling mode.
- smp_send_stop() ensures exclusive hardware access by quiescing other
  CPUs, preventing them from holding bus locks (e.g., I2C spinlocks)
  during the final restart phase.

Signed-off-by: Troy Mitchell <troy.mitchell at linux.dev>
---
 arch/riscv/kernel/reset.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/riscv/kernel/reset.c b/arch/riscv/kernel/reset.c
index 912288572226..7a5dcfdc3674 100644
--- a/arch/riscv/kernel/reset.c
+++ b/arch/riscv/kernel/reset.c
@@ -5,6 +5,7 @@
 
 #include <linux/reboot.h>
 #include <linux/pm.h>
+#include <linux/smp.h>
 
 static void default_power_off(void)
 {
@@ -17,6 +18,10 @@ EXPORT_SYMBOL(pm_power_off);
 
 void machine_restart(char *cmd)
 {
+	/* Disable interrupts first */
+	local_irq_disable();
+	smp_send_stop();
+
 	do_kernel_restart(cmd);
 	while (1);
 }

---
base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
change-id: 20260311-v7-0-rc1-rv-dis-int-before-restart-5b3e52a4b419

Best regards,
-- 
Troy Mitchell <troy.mitchell at linux.dev>