locking: Remove rcu_read_{,un}lock() for preempt_{dis,en}able()
preempt_disable/enable() is equal to RCU read-side crital section, and
the spinning codes in mutex and rwsem could ensure that the preemption
is disabled. So let's remove the unnecessary rcu_read_lock/unlock for
saving some cycles in hot codes.
Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20211013134154.1085649-2-yanfei.xu@windriver.com
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 29eea50..884aa08 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -635,7 +635,10 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
}
preempt_disable();
- rcu_read_lock();
+ /*
+ * Disable preemption is equal to the RCU read-side crital section,
+ * thus the task_strcut structure won't go away.
+ */
owner = rwsem_owner_flags(sem, &flags);
/*
* Don't check the read-owner as the entry may be stale.
@@ -643,7 +646,6 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
if ((flags & RWSEM_NONSPINNABLE) ||
(owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
ret = false;
- rcu_read_unlock();
preempt_enable();
lockevent_cond_inc(rwsem_opt_fail, !ret);
@@ -671,12 +673,13 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
unsigned long flags, new_flags;
enum owner_state state;
+ lockdep_assert_preemption_disabled();
+
owner = rwsem_owner_flags(sem, &flags);
state = rwsem_owner_state(owner, flags);
if (state != OWNER_WRITER)
return state;
- rcu_read_lock();
for (;;) {
/*
* When a waiting writer set the handoff flag, it may spin
@@ -694,7 +697,9 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
* Ensure we emit the owner->on_cpu, dereference _after_
* checking sem->owner still matches owner, if that fails,
* owner might point to free()d memory, if it still matches,
- * the rcu_read_lock() ensures the memory stays valid.
+ * our spinning context already disabled preemption which is
+ * equal to RCU read-side crital section ensures the memory
+ * stays valid.
*/
barrier();
@@ -705,7 +710,6 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
cpu_relax();
}
- rcu_read_unlock();
return state;
}