linux-kernel - [PATCH 47/51] profile: Fix CPU hotplug callback registration

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Message-ID: <20140205221310.19080.72463.stgit@srivatsabhat.in.ibm.com>
Date:	Thu, 06 Feb 2014 03:43:10 +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,
	Al Viro <viro@...iv.linux.org.uk>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH 47/51] profile: 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 profile code by using this latter form of callback registration.

Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Mauro Carvalho Chehab <mchehab@...hat.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 kernel/profile.c |   20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/kernel/profile.c b/kernel/profile.c
index 6631e1e..df27769 100644
--- a/kernel/profile.c
+++ b/kernel/profile.c
@@ -591,18 +591,28 @@ out_cleanup:
 int __ref create_proc_profile(void) /* false positive from hotcpu_notifier */
 {
 	struct proc_dir_entry *entry;
+	int err = 0;
 
 	if (!prof_on)
 		return 0;
-	if (create_hash_tables())
-		return -ENOMEM;
+
+	cpu_maps_update_begin();
+
+	if (create_hash_tables()) {
+		err = -ENOMEM;
+		goto out;
+	}
+
 	entry = proc_create("profile", S_IWUSR | S_IRUGO,
 			    NULL, &proc_profile_operations);
 	if (!entry)
-		return 0;
+		goto out;
 	proc_set_size(entry, (1 + prof_len) * sizeof(atomic_t));
-	hotcpu_notifier(profile_cpu_callback, 0);
-	return 0;
+	__hotcpu_notifier(profile_cpu_callback, 0);
+
+out:
+	cpu_maps_update_done();
+	return err;
 }
 module_init(create_proc_profile);
 #endif /* CONFIG_PROC_FS */

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