linux-kernel - [PATCH 11/51] s390, smp: Fix CPU hotplug callback registration

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Message-ID: <20140205220632.19080.94177.stgit@srivatsabhat.in.ibm.com>
Date:	Thu, 06 Feb 2014 03:36:33 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de, akpm@...ux-foundation.org
Cc:	mingo@...nel.org, paulmck@...ux.vnet.ibm.com, tj@...nel.org,
	walken@...gle.com, ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-s390@...r.kernel.org,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH 11/51] s390, smp: Fix CPU hotplug callback registration

Subsystems that want to register CPU hotplug callbacks, as well as perform
initialization for the CPUs that are already online, often do it as shown
below:

	get_online_cpus();

	for_each_online_cpu(cpu)
		init_cpu(cpu);

	register_cpu_notifier(&foobar_cpu_notifier);

	put_online_cpus();

This is wrong, since it is prone to ABBA deadlocks involving the
cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
with CPU hotplug operations).

Instead, the correct and race-free way of performing the callback
registration is:

	cpu_maps_update_begin();

	for_each_online_cpu(cpu)
		init_cpu(cpu);

	/* Note the use of the double underscored version of the API */
	__register_cpu_notifier(&foobar_cpu_notifier);

	cpu_maps_update_done();


Fix the smp code in s390 by using this latter form of callback registration.

Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Heiko Carstens <heiko.carstens@...ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-s390@...r.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 arch/s390/kernel/smp.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index a7125b6..415a44e 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -1057,19 +1057,24 @@ static DEVICE_ATTR(rescan, 0200, NULL, rescan_store);
 
 static int __init s390_smp_init(void)
 {
-	int cpu, rc;
+	int cpu, rc = 0;
 
-	hotcpu_notifier(smp_cpu_notify, 0);
 #ifdef CONFIG_HOTPLUG_CPU
 	rc = device_create_file(cpu_subsys.dev_root, &dev_attr_rescan);
 	if (rc)
 		return rc;
 #endif
+	cpu_maps_update_begin();
 	for_each_present_cpu(cpu) {
 		rc = smp_add_present_cpu(cpu);
 		if (rc)
-			return rc;
+			goto out;
 	}
-	return 0;
+
+	__hotcpu_notifier(smp_cpu_notify, 0);
+
+out:
+	cpu_maps_update_done();
+	return rc;
 }
 subsys_initcall(s390_smp_init);

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