[PATCH] powerpc/rtas: retry when cpu offline races with suspend/migration

[Nathan Lynch]

The protocol for suspending or migrating an LPAR requires all present
processor threads to enter H_JOIN. So if we have threads offline, we
have to temporarily bring them up. This can race with administrator
actions such as SMT state changes. As of dfd718a2ed1f ("powerpc/rtas:
Fix a potential race between CPU-Offline & Migration"),
rtas_ibm_suspend_me() accounts for this, but errors out with -EBUSY
for what almost certainly is a transient condition in any reasonable
scenario.

Callers of rtas_ibm_suspend_me() already retry when -EAGAIN is
returned, and it is typical during a migration for that to happen
repeatedly for several minutes polling the H_VASI_STATE hcall result
before proceeding to the next stage.

So return -EAGAIN instead of -EBUSY when this race is
encountered. Additionally: logging this event is still appropriate but
use pr_info instead of pr_err; and remove use of unlikely() while here
as this is not a hot path at all.

Fixes: dfd718a2ed1f ("powerpc/rtas: Fix a potential race between CPU-Offline & Migration")
Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
---
 arch/powerpc/kernel/rtas.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
index fbc676160adf..9b4d2a2ffb4f 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -984,10 +984,9 @@ int rtas_ibm_suspend_me(u64 handle)
 	cpu_hotplug_disable();
 
 	/* Check if we raced with a CPU-Offline Operation */
-	if (unlikely(!cpumask_equal(cpu_present_mask, cpu_online_mask))) {
-		pr_err("%s: Raced against a concurrent CPU-Offline\n",
-		       __func__);
-		atomic_set(&data.error, -EBUSY);
+	if (!cpumask_equal(cpu_present_mask, cpu_online_mask)) {
+		pr_info("%s: Raced against a concurrent CPU-Offline\n", __func__);
+		atomic_set(&data.error, -EAGAIN);
 		goto out_hotplug_enable;
 	}
 
-- 
2.20.1