linux-kernel - [PATCH AUTOSEL 5.2 094/171] powerpc/rtas: retry when cpu offline races with suspend/migration

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Date:   Thu, 18 Jul 2019 23:55:25 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Nathan Lynch <nathanl@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sasha Levin <sashal@...nel.org>, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 5.2 094/171] powerpc/rtas: retry when cpu offline races with suspend/migration

From: Nathan Lynch <nathanl@...ux.ibm.com>

[ Upstream commit 9fb603050ffd94f8127df99c699cca2f575eb6a0 ]

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@...ux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 b824f4c69622..fff2eb22427d 100644
--- a/arch/powerpc/kernel/rtas.c
+++ b/arch/powerpc/kernel/rtas.c
@@ -980,10 +980,9 @@ 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