linux-kernel - [PATCH 2/4] cpu/hotplug: CPUHP_BRINGUP

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Message-Id: <1610385047-92151-3-git-send-email-vincent.donnefort@arm.com>
Date:   Mon, 11 Jan 2021 17:10:45 +0000
From:   vincent.donnefort@....com
To:     peterz@...radead.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org
Cc:     valentin.schneider@....com,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: [PATCH 2/4] cpu/hotplug: CPUHP_BRINGUP_CPU exception in fail injection

From: Vincent Donnefort <vincent.donnefort@....com>

The atomic states (between CPUHP_AP_IDLE_DEAD and CPUHP_AP_ONLINE) are
triggered by the CPUHP_BRINGUP_CPU step. If the latter doesn't run, none
of the atomic can. Hence, rollback is not possible after a hotunplug
CPUHP_BRINGUP_CPU step failure and the "fail" interface shouldn't allow
it. Moreover, the current CPUHP_BRINGUP_CPU teardown callback
(finish_cpu()) cannot fail anyway.

Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
---
 kernel/cpu.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 9121edf..bcd7b2a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2216,9 +2216,14 @@ static ssize_t write_cpuhp_fail(struct device *dev,
 		return -EINVAL;
 
 	/*
-	 * Cannot fail STARTING/DYING callbacks.
+	 * Cannot fail STARTING/DYING callbacks. Also, those callbacks are
+	 * triggered by BRINGUP_CPU bringup callback. Therefore, the latter
+	 * can't fail during hotunplug, as it would mean we have no way of
+	 * rolling back the atomic states that have been previously teared
+	 * down.
 	 */
-	if (cpuhp_is_atomic_state(fail))
+	if (cpuhp_is_atomic_state(fail) ||
+	    (fail == CPUHP_BRINGUP_CPU && st->state > CPUHP_BRINGUP_CPU))
 		return -EINVAL;
 
 	/*
-- 
2.7.4