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 oprofile code in x86 by using this latter form of callback
registration. But retain the calls to get/put_online_cpus(), since they are
used in other places as well, to protect the variables 'nmi_enabled' and
'ctr_running'. Strictly speaking, this is not necessary since
cpu_notifier_register_begin/done() provide a stronger synchronization
with CPU hotplug than get/put_online_cpus(). However, let's retain the
calls to get/put_online_cpus() to be consistent with the other call-sites.
By nesting get/put_online_cpus() *inside* cpu_notifier_register_begin/done(),
we avoid the ABBA deadlock possibility mentioned above.
Cc: Robert Richter <r...@kernel.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: x...@kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
---
arch/x86/oprofile/nmi_int.c | 15 +++++++++++++--
1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/arch/x86/oprofile/nmi_int.c b/arch/x86/oprofile/nmi_int.c
index 6890d84..379e8bd 100644
--- a/arch/x86/oprofile/nmi_int.c
+++ b/arch/x86/oprofile/nmi_int.c
@@ -494,14 +494,19 @@ static int nmi_setup(void)
if (err)
goto fail;
+ cpu_notifier_register_begin();
+
+ /* Use get/put_online_cpus() to protect 'nmi_enabled' */
get_online_cpus();
- register_cpu_notifier(&oprofile_cpu_nb);
nmi_enabled = 1;
/* make nmi_enabled visible to the nmi handler: */
smp_mb();
on_each_cpu(nmi_cpu_setup, NULL, 1);
+ __register_cpu_notifier(&oprofile_cpu_nb);
put_online_cpus();
+ cpu_notifier_register_done();
+
return 0;
fail:
free_msrs();
@@ -512,12 +517,18 @@ static void nmi_shutdown(void)
{
struct op_msrs *msrs;
+ cpu_notifier_register_begin();
+
+ /* Use get/put_online_cpus() to protect 'nmi_enabled' & 'ctr_running' */
get_online_cpus();
- unregister_cpu_notifier(&oprofile_cpu_nb);
on_each_cpu(nmi_cpu_shutdown, NULL, 1);
nmi_enabled = 0;
ctr_running = 0;
+ __unregister_cpu_notifier(&oprofile_cpu_nb);
put_online_cpus();
+
+ cpu_notifier_register_done();
+
/* make variables visible to the nmi handler: */
smp_mb();
unregister_nmi_handler(NMI_LOCAL, "oprofile");
--
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