[PATCH 1/3] kernel/reboot: Use the static sys-off handler for any priority

[Samuel Holland]

commit 587b9bfe0668 ("kernel/reboot: Use static handler for
register_platform_power_off()") addded a statically-allocated handler
so register_sys_off_handler() could be called before the slab allocator
is available.

That behavior was limited to the SYS_OFF_PRIO_PLATFORM priority.
However, it is also required for handlers such as PSCI on ARM and SBI on
RISC-V, which should be registered at SYS_OFF_PRIO_FIRMWARE. Currently,
this call stack crashes:

  start_kernel()
    setup_arch()
      psci_dt_init()
        psci_0_2_init()
          register_sys_off_handler()
            kmem_cache_alloc()

Generalize the code to use the statically-allocated handler for the
first registration, regardless of priority. Check .sys_off_cb for
conflicts instead of .cb_data; some callbacks (e.g. firmware drivers)
do not need any per-instance data, so .cb_data could be NULL.

Reviewed-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
Signed-off-by: Samuel Holland <samuel at sholland.org>
---

 kernel/reboot.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/kernel/reboot.c b/kernel/reboot.c
index 3bba88c7ffc6..38c18d4f0a36 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -327,7 +327,7 @@ static int sys_off_notify(struct notifier_block *nb,
 	return handler->sys_off_cb(&data);
 }
 
-static struct sys_off_handler platform_sys_off_handler;
+static struct sys_off_handler early_sys_off_handler;
 
 static struct sys_off_handler *alloc_sys_off_handler(int priority)
 {
@@ -338,10 +338,8 @@ static struct sys_off_handler *alloc_sys_off_handler(int priority)
 	 * Platforms like m68k can't allocate sys_off handler dynamically
 	 * at the early boot time because memory allocator isn't available yet.
 	 */
-	if (priority == SYS_OFF_PRIO_PLATFORM) {
-		handler = &platform_sys_off_handler;
-		if (handler->cb_data)
-			return ERR_PTR(-EBUSY);
+	if (!early_sys_off_handler.sys_off_cb) {
+		handler = &early_sys_off_handler;
 	} else {
 		if (system_state > SYSTEM_RUNNING)
 			flags = GFP_ATOMIC;
@@ -358,7 +356,7 @@ static struct sys_off_handler *alloc_sys_off_handler(int priority)
 
 static void free_sys_off_handler(struct sys_off_handler *handler)
 {
-	if (handler == &platform_sys_off_handler)
+	if (handler == &early_sys_off_handler)
 		memset(handler, 0, sizeof(*handler));
 	else
 		kfree(handler);
-- 
2.37.4