[PATCH v2 07/18] sched: tasks: Use sequence counter with associated spinlock

[Ahmed S. Darwish]

A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.

Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.

If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.

Signed-off-by: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>
---
 include/linux/sched.h | 2 +-
 init/init_task.c      | 3 ++-
 kernel/fork.c         | 2 +-
 3 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..a9ce6fbeb735 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1046,7 +1046,7 @@ struct task_struct {
 	/* Protected by ->alloc_lock: */
 	nodemask_t			mems_allowed;
 	/* Seqence number to catch updates: */
-	seqcount_t			mems_allowed_seq;
+	seqcount_spinlock_t		mems_allowed_seq;
 	int				cpuset_mem_spread_rotor;
 	int				cpuset_slab_spread_rotor;
 #endif
diff --git a/init/init_task.c b/init/init_task.c
index bd403ed3e418..94bf4aea8293 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -142,7 +142,8 @@ struct task_struct init_task
 	.rcu_tasks_idle_cpu = -1,
 #endif
 #ifdef CONFIG_CPUSETS
-	.mems_allowed_seq = SEQCNT_ZERO(init_task.mems_allowed_seq),
+	.mems_allowed_seq = SEQCNT_SPINLOCK_ZERO(init_task.mems_allowed_seq,
+						 &init_task.alloc_lock),
 #endif
 #ifdef CONFIG_RT_MUTEXES
 	.pi_waiters	= RB_ROOT_CACHED,
diff --git a/kernel/fork.c b/kernel/fork.c
index 48ed22774efa..3b88bef92875 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2019,7 +2019,7 @@ static __latent_entropy struct task_struct *copy_process(
 #ifdef CONFIG_CPUSETS
 	p->cpuset_mem_spread_rotor = NUMA_NO_NODE;
 	p->cpuset_slab_spread_rotor = NUMA_NO_NODE;
-	seqcount_init(&p->mems_allowed_seq);
+	seqcount_spinlock_init(&p->mems_allowed_seq, &p->alloc_lock);
 #endif
 #ifdef CONFIG_TRACE_IRQFLAGS
 	p->irq_events = 0;
-- 
2.20.1