From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Mon, 20 Jul 2020 17:55:19 +0200
Subject: sched: tasks: Use sequence counter with associated spinlock
Patch-mainline: v5.9-rc1
Git-commit: b75058614fdd3140074a640b514f6a0b4d485a2d
References: bsc#1176564 bsc#1162702

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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-14-a.darwish@linutronix.de
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/sched.h |    2 +-
 init/init_task.c      |    3 ++-
 kernel/fork.c         |    2 +-
 3 files changed, 4 insertions(+), 3 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1027,7 +1027,7 @@ 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
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -141,7 +141,8 @@ 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,
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1937,7 +1937,7 @@ static __latent_entropy struct task_stru
 #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;