[PATCH 3.16 16/72] powerpc/rtas: use device model APIs and serialization during LPM

[Ben Hutchings]

3.16.79-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Nathan Lynch <nathanl@xxxxxxxxxxxxx>

commit a6717c01ddc259f6f73364779df058e2c67309f8 upstream.

The LPAR migration implementation and userspace-initiated cpu hotplug
can interleave their executions like so:

1. Set cpu 7 offline via sysfs.

2. Begin a partition migration, whose implementation requires the OS
   to ensure all present cpus are online; cpu 7 is onlined:

     rtas_ibm_suspend_me -> rtas_online_cpus_mask -> cpu_up

   This sets cpu 7 online in all respects except for the cpu's
   corresponding struct device; dev->offline remains true.

3. Set cpu 7 online via sysfs. _cpu_up() determines that cpu 7 is
   already online and returns success. The driver core (device_online)
   sets dev->offline = false.

4. The migration completes and restores cpu 7 to offline state:

     rtas_ibm_suspend_me -> rtas_offline_cpus_mask -> cpu_down

This leaves cpu7 in a state where the driver core considers the cpu
device online, but in all other respects it is offline and
unused. Attempts to online the cpu via sysfs appear to succeed but the
driver core actually does not pass the request to the lower-level
cpuhp support code. This makes the cpu unusable until the cpu device
is manually set offline and then online again via sysfs.

Instead of directly calling cpu_up/cpu_down, the migration code should
use the higher-level device core APIs to maintain consistent state and
serialize operations.

Fixes: 120496ac2d2d ("powerpc: Bring all threads online prior to migration/hibernation")
Signed-off-by: Nathan Lynch <nathanl@xxxxxxxxxxxxx>
Reviewed-by: Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20190802192926.19277-2-nathanl@xxxxxxxxxxxxx
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/powerpc/kernel/rtas.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -855,15 +855,17 @@ static int rtas_cpu_state_change_mask(en
 		return 0;
 
 	for_each_cpu(cpu, cpus) {
+		struct device *dev = get_cpu_device(cpu);
+
 		switch (state) {
 		case DOWN:
-			cpuret = cpu_down(cpu);
+			cpuret = device_offline(dev);
 			break;
 		case UP:
-			cpuret = cpu_up(cpu);
+			cpuret = device_online(dev);
 			break;
 		}
-		if (cpuret) {
+		if (cpuret < 0) {
 			pr_debug("%s: cpu_%s for cpu#%d returned %d.\n",
 					__func__,
 					((state == UP) ? "up" : "down"),
@@ -955,6 +957,8 @@ int rtas_ibm_suspend_me(struct rtas_args
 	data.token = rtas_token("ibm,suspend-me");
 	data.complete = &done;
 
+	lock_device_hotplug();
+
 	/* All present CPUs must be online */
 	cpumask_andnot(offline_mask, cpu_present_mask, cpu_online_mask);
 	cpuret = rtas_online_cpus_mask(offline_mask);
@@ -986,6 +990,7 @@ int rtas_ibm_suspend_me(struct rtas_args
 				__func__);
 
 out:
+	unlock_device_hotplug();
 	free_cpumask_var(offline_mask);
 	return atomic_read(&data.error);
 }