linux-kernel - [PATCH 09/51] arm, kvm: Fix CPU hotplug callback registration

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Message-ID: <20140205220613.19080.58823.stgit@srivatsabhat.in.ibm.com>
Date:	Thu, 06 Feb 2014 03:36:14 +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,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH 09/51] arm, kvm: 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 kvm code in arm by using this latter form of callback registration.

Cc: Christoffer Dall <christoffer.dall@...aro.org>
Cc: Gleb Natapov <gleb@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Russell King <linux@....linux.org.uk>
Cc: kvmarm@...ts.cs.columbia.edu
Cc: kvm@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 arch/arm/kvm/arm.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
index 1d8248e..e2ef4c4 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -1050,21 +1050,26 @@ int kvm_arch_init(void *opaque)
 		}
 	}
 
+	cpu_maps_update_begin();
+
 	err = init_hyp_mode();
 	if (err)
 		goto out_err;
 
-	err = register_cpu_notifier(&hyp_init_cpu_nb);
+	err = __register_cpu_notifier(&hyp_init_cpu_nb);
 	if (err) {
 		kvm_err("Cannot register HYP init CPU notifier (%d)\n", err);
 		goto out_err;
 	}
 
+	cpu_maps_update_done();
+
 	hyp_cpu_pm_init();
 
 	kvm_coproc_table_init();
 	return 0;
 out_err:
+	cpu_maps_update_done();
 	return err;
 }
 

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