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From: Sven Schnelle <svens@linux.ibm.com>
To: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH] sched/core: Test online status in available_idle_cpu()
Date: Mon, 29 Apr 2024 07:54:15 +0200
Message-Id: <20240429055415.3278672-1-svens@linux.ibm.com>
Precedence: bulk
MIME-Version: 1.0
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The current implementation of available_idle_cpu() doesn't test
whether a possible cpu is offline. On s390 this dereferences a
NULL pointer in arch_vcpu_is_preempted() because lowcore is not
allocated for offline cpus. On x86, tracing also shows calls to
available_idle_cpu() after a cpu is disabled, but it looks like
this isn't causing any (obvious) issue. Nevertheless, add a check
and return early if the cpu isn't online.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7019a40457a6..1d9b80411594 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7434,6 +7434,9 @@ int idle_cpu(int cpu)
  */
 int available_idle_cpu(int cpu)
 {
+	if (!cpu_online(cpu))
+		return 0;
+
 	if (!idle_cpu(cpu))
 		return 0;
 
-- 
2.40.1