linux-kernel - Re: [PATCH 3/6] locking/rwsem: Rework writer wakeup

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Date:   Sun, 26 Feb 2023 16:04:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     longman@...hat.com, mingo@...hat.com, will@...nel.org
Cc:     linux-kernel@...r.kernel.org, boqun.feng@...il.com
Subject: Re: [PATCH 3/6] locking/rwsem: Rework writer wakeup

On Thu, Feb 23, 2023 at 01:26:45PM +0100, Peter Zijlstra wrote:
> @@ -1072,7 +1067,7 @@ rwsem_down_read_slowpath(struct rw_semap
>  	for (;;) {
>  		set_current_state(state);
>  		if (!smp_load_acquire(&waiter.task)) {
> -			/* Matches rwsem_mark_wake()'s smp_store_release(). */
> +			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
>  			break;
>  		}
>  		if (signal_pending_state(state, current)) {
> @@ -1143,54 +1138,36 @@ rwsem_down_write_slowpath(struct rw_sema
>  	} else {
>  		atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);

Found it; if we remove the try_write_lock below, then at least this
new-waiter path needs to still do a trylock.

Let me go test the other patches on top of all this and push out a fresh
set if that all still works.

>  	}
> +	raw_spin_unlock_irq(&sem->wait_lock);
>  
>  	/* wait until we successfully acquire the lock */
> -	set_current_state(state);
>  	trace_contention_begin(sem, LCB_F_WRITE);
>  
>  	for (;;) {
> -		if (rwsem_try_write_lock(sem, &waiter)) {
> -			/* rwsem_try_write_lock() implies ACQUIRE on success */
> +		set_current_state(state);
> +		if (!smp_load_acquire(&waiter.task)) {
> +			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
>  			break;
>  		}
> -
> -		raw_spin_unlock_irq(&sem->wait_lock);
> -
> -		if (signal_pending_state(state, current))
> -			goto out_nolock;
> -
> -		/*
> -		 * After setting the handoff bit and failing to acquire
> -		 * the lock, attempt to spin on owner to accelerate lock
> -		 * transfer. If the previous owner is a on-cpu writer and it
> -		 * has just released the lock, OWNER_NULL will be returned.
> -		 * In this case, we attempt to acquire the lock again
> -		 * without sleeping.
> -		 */
> -		if (waiter.handoff_set) {
> -			enum owner_state owner_state;
> -
> -			owner_state = rwsem_spin_on_owner(sem);
> -			if (owner_state == OWNER_NULL)
> -				goto trylock_again;
> +		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(state);
> -trylock_again:
> -		raw_spin_lock_irq(&sem->wait_lock);
>  	}
>  	__set_current_state(TASK_RUNNING);
> -	raw_spin_unlock_irq(&sem->wait_lock);
>  	lockevent_inc(rwsem_wlock);
>  	trace_contention_end(sem, 0);
>  	return sem;
>  
>  out_nolock:
> -	__set_current_state(TASK_RUNNING);
> -	raw_spin_lock_irq(&sem->wait_lock);
>  	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);
> 
>