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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
This commit is contained in:
Yanfei Xu 2021-10-13 21:41:52 +08:00 committed by Peter Zijlstra
parent 7cdacc5f52
commit 6c2787f2a2
2 changed files with 24 additions and 12 deletions
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22
kernel/locking/mutex.c
View File

@ -351,13 +351,16 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
{
bool ret = true;
rcu_read_lock();
lockdep_assert_preemption_disabled();
while (__mutex_owner(lock) == owner) {
/*
* Ensure we emit the owner->on_cpu, dereference _after_
* checking lock->owner still matches owner. If that fails,
* owner might point to freed memory. If it still matches,
* the rcu_read_lock() ensures the memory stays valid.
* checking lock->owner still matches owner. And we already
* disabled preemption which is equal to the RCU read-side
* crital section in optimistic spinning code. Thus the
* task_strcut structure won't go away during the spinning
* period
*/
barrier();
@ -377,7 +380,6 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
cpu_relax();
}
rcu_read_unlock();
return ret;
}
@ -390,19 +392,25 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
struct task_struct *owner;
int retval = 1;
lockdep_assert_preemption_disabled();
if (need_resched())
return 0;
rcu_read_lock();
/*
* We already disabled preemption which is equal to the RCU read-side
* crital section in optimistic spinning code. Thus the task_strcut
* structure won't go away during the spinning period.
*/
owner = __mutex_owner(lock);
/*
* As lock holder preemption issue, we both skip spinning if task is not
* on cpu or its cpu is preempted
*/
if (owner)
retval = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
rcu_read_unlock();
/*
* If lock->owner is not set, the mutex has been released. Return true

14
kernel/locking/rwsem.c
View File

@ -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;
}