md: raid5: Make raid5_percpu handling RT aware

__raid_run_ops() disables preemption with get_cpu() around the access
to the raid5_percpu variables. That causes scheduling while atomic
spews on RT.

Serialize the access to the percpu data with a lock and keep the code
preemptible.

Reported-by: Udo van den Heuvel <udovdh@xs4all.nl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Udo van den Heuvel <udovdh@xs4all.nl>

drivers/md/raid5.c

@@ -2067,8 +2067,9 @@ static void raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
  struct raid5_percpu *percpu;
  unsigned long cpu;
 
- cpu = get_cpu();
+ cpu = get_cpu_light();
  percpu = per_cpu_ptr(conf->percpu, cpu);
+ spin_lock(&percpu->lock);
  if (test_bit(STRIPE_OP_BIOFILL, &ops_request)) {
  ops_run_biofill(sh);
  overlap_clear++;
@@ -2127,7 +2128,8 @@ static void raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
  if (test_and_clear_bit(R5_Overlap, &dev->flags))
  wake_up(&sh->raid_conf->wait_for_overlap);
  }
- put_cpu();
+ spin_unlock(&percpu->lock);
+ put_cpu_light();
 }
 
 static void free_stripe(struct kmem_cache *sc, struct stripe_head *sh)
@@ -6796,6 +6798,7 @@ static int raid456_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
  __func__, cpu);
  return -ENOMEM;
  }
+ spin_lock_init(&per_cpu_ptr(conf->percpu, cpu)->lock);
  return 0;
 }
 
@@ -6806,7 +6809,6 @@ static int raid5_alloc_percpu(struct r5conf *conf)
  conf->percpu = alloc_percpu(struct raid5_percpu);
  if (!conf->percpu)
  return -ENOMEM;
-
  err = cpuhp_state_add_instance(CPUHP_MD_RAID5_PREPARE, &conf->node);
  if (!err) {
  conf->scribble_disks = max(conf->raid_disks,

drivers/md/raid5.h

@@ -624,6 +624,7 @@ struct r5conf {
  int recovery_disabled;
  /* per cpu variables */
  struct raid5_percpu {
+ spinlock_t lock; /* Protection for -RT */
  struct page *spare_page; /* Used when checking P/Q in raid6 */
  struct flex_array *scribble; /* space for constructing buffer
  * lists and performing address