[v3,09/52] arm, hw-breakpoint: Fix CPU hotplug callback registration

Commit Message

Srivatsa S. Bhat March 10, 2014, 8:35 p.m. UTC

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_notifier_register_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_notifier_register_done();


Fix the hw-breakpoint code in arm by using this latter form of callback
registration.

Cc: Russell King <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: [e-mail]
Acked-by: Will Deacon <[e-mail]>
Signed-off-by: Srivatsa S. Bhat <[e-mail]>
---

 arch/arm/kernel/hw_breakpoint.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)


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Patch

diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
index 3d44660..3702de8 100644
--- a/arch/arm/kernel/hw_breakpoint.c
+++ b/arch/arm/kernel/hw_breakpoint.c
@@ -1072,6 +1072,8 @@  static int __init arch_hw_breakpoint_init(void)
 	core_num_brps = get_num_brps();
 	core_num_wrps = get_num_wrps();
 
+	cpu_notifier_register_begin();
+
 	/*
 	 * We need to tread carefully here because DBGSWENABLE may be
 	 * driven low on this core and there isn't an architected way to
@@ -1088,6 +1090,7 @@  static int __init arch_hw_breakpoint_init(void)
 	if (!cpumask_empty(&debug_err_mask)) {
 		core_num_brps = 0;
 		core_num_wrps = 0;
+		cpu_notifier_register_done();
 		return 0;
 	}
 
@@ -1107,7 +1110,10 @@  static int __init arch_hw_breakpoint_init(void)
 			TRAP_HWBKPT, "breakpoint debug exception");
 
 	/* Register hotplug and PM notifiers. */
-	register_cpu_notifier(&dbg_reset_nb);
+	__register_cpu_notifier(&dbg_reset_nb);
+
+	cpu_notifier_register_done();
+
 	pm_init();
 	return 0;
 }