From: Oleg Nesterov <oleg@xxxxxxxxxx>
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).
Interestingly, the raid5 code can actually prevent double initialization and
hence can use the following simplified form of callback registration:
register_cpu_notifier(&foobar_cpu_notifier);
get_online_cpus();
for_each_online_cpu(cpu)
init_cpu(cpu);
put_online_cpus();
A hotplug operation that occurs between registering the notifier and calling
get_online_cpus(), won't disrupt anything, because the code takes care to
perform the memory allocations only once.
So reorganize the code in raid5 this way to fix the deadlock with callback
registration.
Cc: Neil Brown <neilb@xxxxxxx>
Cc: linux-raid@xxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
[Srivatsa: Fixed the unregister_cpu_notifier() deadlock, added the
free_scratch_buffer() helper to condense code further and wrote the changelog.]
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---
drivers/md/raid5.c | 90 +++++++++++++++++++++++++---------------------------
1 file changed, 44 insertions(+), 46 deletions(-)
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index f1feade..16f5c21 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -5514,23 +5514,43 @@ raid5_size(struct mddev *mddev, sector_t sectors, int raid_disks)
return sectors * (raid_disks - conf->max_degraded);
}
+static void free_scratch_buffer(struct r5conf *conf, struct raid5_percpu *percpu)
+{
+ safe_put_page(percpu->spare_page);
+ kfree(percpu->scribble);
+ percpu->spare_page = NULL;
+ percpu->scribble = NULL;
+}
+
+static int alloc_scratch_buffer(struct r5conf *conf, struct raid5_percpu *percpu)
+{
+ if (conf->level == 6 && !percpu->spare_page)
+ percpu->spare_page = alloc_page(GFP_KERNEL);
+ if (!percpu->scribble)
+ percpu->scribble = kmalloc(conf->scribble_len, GFP_KERNEL);
+
+ if (!percpu->scribble || (conf->level == 6 && !percpu->spare_page)) {
+ free_scratch_buffer(conf, percpu);
+ return -ENOMEM;
+ }
+
+ return 0;
+}
+
static void raid5_free_percpu(struct r5conf *conf)
{
- struct raid5_percpu *percpu;
unsigned long cpu;
if (!conf->percpu)
return;
- get_online_cpus();
- for_each_possible_cpu(cpu) {
- percpu = per_cpu_ptr(conf->percpu, cpu);
- safe_put_page(percpu->spare_page);
- kfree(percpu->scribble);
- }
#ifdef CONFIG_HOTPLUG_CPU
unregister_cpu_notifier(&conf->cpu_notify);
#endif
+
+ get_online_cpus();
+ for_each_possible_cpu(cpu)
+ free_scratch_buffer(conf, per_cpu_ptr(conf->percpu, cpu));
put_online_cpus();
free_percpu(conf->percpu);
@@ -5557,15 +5577,7 @@ static int raid456_cpu_notify(struct notifier_block *nfb, unsigned long action,
switch (action) {
case CPU_UP_PREPARE:
case CPU_UP_PREPARE_FROZEN:
- if (conf->level == 6 && !percpu->spare_page)
- percpu->spare_page = alloc_page(GFP_KERNEL);
- if (!percpu->scribble)
- percpu->scribble = kmalloc(conf->scribble_len, GFP_KERNEL);
-
- if (!percpu->scribble ||
- (conf->level == 6 && !percpu->spare_page)) {
- safe_put_page(percpu->spare_page);
- kfree(percpu->scribble);
+ if (alloc_scratch_buffer(conf, percpu)) {
pr_err("%s: failed memory allocation for cpu%ld\n",
__func__, cpu);
return notifier_from_errno(-ENOMEM);
@@ -5573,10 +5585,7 @@ static int raid456_cpu_notify(struct notifier_block *nfb, unsigned long action,
break;
case CPU_DEAD:
case CPU_DEAD_FROZEN:
- safe_put_page(percpu->spare_page);
- kfree(percpu->scribble);
- percpu->spare_page = NULL;
- percpu->scribble = NULL;
+ free_scratch_buffer(conf, per_cpu_ptr(conf->percpu, cpu));
break;
default:
break;
@@ -5588,40 +5597,29 @@ static int raid456_cpu_notify(struct notifier_block *nfb, unsigned long action,
static int raid5_alloc_percpu(struct r5conf *conf)
{
unsigned long cpu;
- struct page *spare_page;
- struct raid5_percpu __percpu *allcpus;
- void *scribble;
- int err;
+ int err = 0;
- allcpus = alloc_percpu(struct raid5_percpu);
- if (!allcpus)
+ conf->percpu = alloc_percpu(struct raid5_percpu);
+ if (!conf->percpu)
return -ENOMEM;
- conf->percpu = allcpus;
+
+#ifdef CONFIG_HOTPLUG_CPU
+ conf->cpu_notify.notifier_call = raid456_cpu_notify;
+ conf->cpu_notify.priority = 0;
+ err = register_cpu_notifier(&conf->cpu_notify);
+ if (err)
+ return err;
+#endif
get_online_cpus();
- err = 0;
for_each_present_cpu(cpu) {
- if (conf->level == 6) {
- spare_page = alloc_page(GFP_KERNEL);
- if (!spare_page) {
- err = -ENOMEM;
- break;
- }
- per_cpu_ptr(conf->percpu, cpu)->spare_page = spare_page;
- }
- scribble = kmalloc(conf->scribble_len, GFP_KERNEL);
- if (!scribble) {
- err = -ENOMEM;
+ err = alloc_scratch_buffer(conf, per_cpu_ptr(conf->percpu, cpu));
+ if (err) {
+ pr_err("%s: failed memory allocation for cpu%ld\n",
+ __func__, cpu);
break;
}
- per_cpu_ptr(conf->percpu, cpu)->scribble = scribble;
}
-#ifdef CONFIG_HOTPLUG_CPU
- conf->cpu_notify.notifier_call = raid456_cpu_notify;
- conf->cpu_notify.priority = 0;
- if (err == 0)
- err = register_cpu_notifier(&conf->cpu_notify);
-#endif
put_online_cpus();
return err;
--
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