linux-kernel - Re: [PATCH 5/6] locking/rwsem: Unify wait loop

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Date:   Thu, 23 Feb 2023 17:33:53 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     longman@...hat.com, mingo@...hat.com, will@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/6] locking/rwsem: Unify wait loop

On Thu, Feb 23, 2023 at 11:31:47AM -0800, Boqun Feng wrote:
[..]
> > +#define waiter_type(_waiter, _r, _w)	\
> > +	((_waiter)->type == RWSEM_WAITING_FOR_READ ? (_r) : (_w))
> > +
> > +static __always_inline struct rw_semaphore *
> > +rwsem_waiter_wait(struct rw_semaphore *sem, struct rwsem_waiter *waiter, int state)
> > +{
> > +	trace_contention_begin(sem, waiter_type(waiter, LCB_F_READ, LCB_F_WRITE));
> > +
> > +	/* wait to be given the lock */
> > +	for (;;) {
> > +		set_current_state(state);
> > +		if (!smp_load_acquire(&waiter->task)) {
> > +			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
> > +			break;
> > +		}
> > +		if (signal_pending_state(state, current)) {
> > +			raw_spin_lock_irq(&sem->wait_lock);
> 
> Move the below __set_current_state(TASK_RUNNING)s up here? I think we
> need the preemption protection when changing the task state here.
> 

Nevermind since we have the preemption protection for the whole
function... but merging two __set_current_state()s into one still looks
good.

Regards,
Boqun

> > +			if (waiter->task)
> > +				goto out_nolock;
> 
[...]
> > +
> > +	__set_current_state(TASK_RUNNING);
> > +	lockevent_inc(waiter_type(waiter, rwsem_rlock, rwsem_wlock));
> > +	trace_contention_end(sem, 0);
> > +	return sem;
> > +
> > +out_nolock:
> > +	rwsem_del_wake_waiter(sem, waiter);
> > +	__set_current_state(TASK_RUNNING);
> > +	lockevent_inc(waiter_type(waiter, rwsem_rlock_fail, rwsem_wlock_fail));
> > +	trace_contention_end(sem, -EINTR);
> > +	return ERR_PTR(-EINTR);
> > +}
> > +
> >  /*
> >   * Wait for the read lock to be granted
> >   */
> > @@ -1071,38 +1109,7 @@ rwsem_down_read_slowpath(struct rw_semap
> >  	if (!wake_q_empty(&wake_q))
> >  		wake_up_q(&wake_q);
> >  
> > -	trace_contention_begin(sem, LCB_F_READ);
> > -
> > -	/* wait to be given the lock */
> > -	for (;;) {
> > -		set_current_state(state);
> > -		if (!smp_load_acquire(&waiter.task)) {
> > -			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
> > -			break;
> > -		}
> > -		if (signal_pending_state(state, current)) {
> > -			raw_spin_lock_irq(&sem->wait_lock);
> > -			if (waiter.task)
> > -				goto out_nolock;
> > -			raw_spin_unlock_irq(&sem->wait_lock);
> > -			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
> > -			break;
> > -		}
> > -		schedule_preempt_disabled();
> > -		lockevent_inc(rwsem_sleep_reader);
> > -	}
> > -
> > -	__set_current_state(TASK_RUNNING);
> > -	lockevent_inc(rwsem_rlock);
> > -	trace_contention_end(sem, 0);
> > -	return sem;
> > -
> > -out_nolock:
> > -	rwsem_del_wake_waiter(sem, &waiter, &wake_q);
> > -	__set_current_state(TASK_RUNNING);
> > -	lockevent_inc(rwsem_rlock_fail);
> > -	trace_contention_end(sem, -EINTR);
> > -	return ERR_PTR(-EINTR);
> > +	return rwsem_waiter_wait(sem, &waiter, state);
> >  }
> >  
> >  /*
> > @@ -1150,37 +1157,7 @@ rwsem_down_write_slowpath(struct rw_sema
> >  	}
> >  	raw_spin_unlock_irq(&sem->wait_lock);
> >  
> > -	/* wait until we successfully acquire the lock */
> > -	trace_contention_begin(sem, LCB_F_WRITE);
> > -
> > -	for (;;) {
> > -		set_current_state(state);
> > -		if (!smp_load_acquire(&waiter.task)) {
> > -			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
> > -			break;
> > -		}
> > -		if (signal_pending_state(state, current)) {
> > -			raw_spin_lock_irq(&sem->wait_lock);
> > -			if (waiter.task)
> > -				goto out_nolock;
> > -			raw_spin_unlock_irq(&sem->wait_lock);
> > -			/* Ordered by sem->wait_lock against rwsem_mark_wake(). */
> > -			break;
> > -		}
> > -		schedule_preempt_disabled();
> > -		lockevent_inc(rwsem_sleep_writer);
> > -	}
> > -	__set_current_state(TASK_RUNNING);
> > -	lockevent_inc(rwsem_wlock);
> > -	trace_contention_end(sem, 0);
> > -	return sem;
> > -
> > -out_nolock:
> > -	rwsem_del_wake_waiter(sem, &waiter, &wake_q);
> > -	__set_current_state(TASK_RUNNING);
> > -	lockevent_inc(rwsem_wlock_fail);
> > -	trace_contention_end(sem, -EINTR);
> > -	return ERR_PTR(-EINTR);
> > +	return rwsem_waiter_wait(sem, &waiter, state);
> >  }
> >  
> >  /*
> > 
> >