linux-kernel - [PATCH v2] cpu/hotplug: Set st->cpu earlier

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Message-Id: <20220316153637.288199-1-steven.price@arm.com>
Date:   Wed, 16 Mar 2022 15:36:37 +0000
From:   Steven Price <steven.price@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Donnefort <vincent.donnefort@....com>
Cc:     linux-kernel@...r.kernel.org, Baokun Li <libaokun1@...wei.com>,
        Dongli Zhang <dongli.zhang@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Yuan ZhaoXiong <yuanzhaoxiong@...du.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Steven Price <steven.price@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: [PATCH v2] cpu/hotplug: Set st->cpu earlier

Setting the 'cpu' member of struct cpuhp_cpu_state in cpuhp_create() is
too late as other callbacks can be made before that point. In particular
if one of the earlier callbacks fails and triggers a rollback that
rollback will be done with st->cpu==0 causing CPU0 to be erroneously set
to be dying, causing the scheduler to get mightily confused and throw
its toys out of the pram.

Move the assignment earlier before any callbacks have a chance to run.

Fixes: 2ea46c6fc945 ("cpumask/hotplug: Fix cpu_dying() state tracking")
Signed-off-by: Steven Price <steven.price@....com>
CC: Dietmar Eggemann <dietmar.eggemann@....com>
---
Changes since v1[1]:

 * Added a Fixes: tag.
 * Moved the assignment to just before cpuhp_set_state() which is the
   first place it is needed.

[1]: https://lore.kernel.org/r/20220225134918.105796-1-steven.price%40arm.com

 kernel/cpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 407a2568f35e..c1324c8677cf 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -720,7 +720,6 @@ static void cpuhp_create(unsigned int cpu)
 
 	init_completion(&st->done_up);
 	init_completion(&st->done_down);
-	st->cpu = cpu;
 }
 
 static int cpuhp_should_run(unsigned int cpu)
@@ -1351,6 +1350,7 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
 
 	cpuhp_tasks_frozen = tasks_frozen;
 
+	st->cpu = cpu;
 	cpuhp_set_state(st, target);
 	/*
 	 * If the current CPU state is in the range of the AP hotplug thread,
-- 
2.25.1