[PATCH v2 09/52] arm, hw-breakpoint: Fix CPU hotplug callback registration

[Srivatsa S. Bhat]

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 <li...@arm.linux.org.uk>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: linux-arm-ker...@lists.infradead.org
Acked-by: Will Deacon <will.dea...@arm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
---

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

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;
 }

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/