linux-kernel - Re: [PATCH v2 8/8] locking/rwsem: Restore old write lock slow path behavior

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Date:   Tue, 28 Mar 2023 23:45:06 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 8/8] locking/rwsem: Restore old write lock slow path
 behavior

On 3/28/23 10:02, Peter Zijlstra wrote:
> On Mon, Mar 27, 2023 at 04:24:13PM -0400, Waiman Long wrote:
>
>>   kernel/locking/rwsem.c | 34 +++++++++++++++++++++++++++++++---
>>   1 file changed, 31 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
>> index 7bd26e64827a..cf9dc1e250e0 100644
>> --- a/kernel/locking/rwsem.c
>> +++ b/kernel/locking/rwsem.c
>> @@ -426,6 +426,7 @@ rwsem_waiter_wake(struct rwsem_waiter *waiter, struct wake_q_head *wake_q)
>>   static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
>>   					struct rwsem_waiter *waiter)
>>   {
>> +	bool first = (rwsem_first_waiter(sem) == waiter);
>>   	long count, new;
>>   
>>   	lockdep_assert_held(&sem->wait_lock);
>> @@ -434,6 +435,9 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
>>   	do {
>>   		new = count;
>>   
>> +		if (!first && (count & (RWSEM_FLAG_HANDOFF | RWSEM_LOCK_MASK)))
>> +			return false;
>> +
>>   		if (count & RWSEM_LOCK_MASK) {
>>   			/*
>>   			 * A waiter (first or not) can set the handoff bit
> I couldn't immediately make sense of the above, and I think there's case
> where not-first would still want to set handoff you're missing.

It is possible to do that, but we need a minor change to make sure that 
you set the handoff_set flag of the first waiter instead of the current 
waiter which is what the current rwsem code is doing. Other than that, I 
think the change is OK, though I need to take a further look into it 
tomorrow as it is now late for me too.

Cheers,
longman

>
> After a few detours, I ended up with the below; does that make sense or
> did I just make a bigger mess? (entirely possible due to insufficient
> sleep etc..).
>
>
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -426,12 +426,27 @@ rwsem_waiter_wake(struct rwsem_waiter *w
>   static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
>   					struct rwsem_waiter *waiter)
>   {
> +	bool first = (rwsem_first_waiter(sem) == waiter);
>   	long count, new;
>   
>   	lockdep_assert_held(&sem->wait_lock);
>   
>   	count = atomic_long_read(&sem->count);
>   	do {
> +		/*
> +		 * first handoff
> +		 *
> +		 *   0     0     | take
> +		 *   0     1     | not take
> +		 *   1     0     | take
> +		 *   1     1     | take
> +		 *
> +		 */
> +		bool handoff = count & RWSEM_FLAG_HANDOFF;
> +
> +		if (!first && handoff)
> +			return false;
> +
>   		new = count;
>   
>   		if (count & RWSEM_LOCK_MASK) {
> @@ -440,7 +455,7 @@ static inline bool rwsem_try_write_lock(
>   			 * if it is an RT task or wait in the wait queue
>   			 * for too long.
>   			 */
> -			if ((count & RWSEM_FLAG_HANDOFF) ||
> +			if (handoff ||
>   			    (!rt_task(waiter->task) &&
>   			     !time_after(jiffies, waiter->timeout)))
>   				return false;
> @@ -501,11 +516,19 @@ static void rwsem_writer_wake(struct rw_
>   		 */
>   		list_del(&waiter->list);
>   		atomic_long_set(&sem->owner, (long)waiter->task);
> -
> -	} else if (!rwsem_try_write_lock(sem, waiter))
> +		rwsem_waiter_wake(waiter, wake_q);
>   		return;
> +	}
>   
> -	rwsem_waiter_wake(waiter, wake_q);
> +	/*
> +	 * Mark writer at the front of the queue for wakeup.
> +	 *
> +	 * Until the task is actually awoken later by the caller, other writers
> +	 * are able to steal it. Readers, on the other hand, will block as they
> +	 * will notice the queued writer.
> +	 */
> +	wake_q_add(wake_q, waiter->task);
> +	lockevent_inc(rwsem_wake_writer);
>   }
>   
>   static void rwsem_reader_wake(struct rw_semaphore *sem,
> @@ -1038,6 +1061,25 @@ rwsem_waiter_wait(struct rw_semaphore *s
>   			/* Matches rwsem_waiter_wake()'s smp_store_release(). */
>   			break;
>   		}
> +		if (!reader) {
> +			/*
> +			 * If rwsem_writer_wake() did not take the lock, we must
> +			 * rwsem_try_write_lock() here.
> +			 */
> +			raw_spin_lock_irq(&sem->wait_lock);
> +			if (waiter->task && rwsem_try_write_lock(sem, waiter)) {
> +				waiter->task = NULL;
> +				raw_spin_unlock_irq(&sem->wait_lock);
> +				break;
> +			}
> +			raw_spin_unlock_irq(&sem->wait_lock);
> +
> +			if (waiter->handoff_set)
> +				rwsem_spin_on_owner(sem);
> +
> +			if (!smp_load_acquire(&waiter->task))
> +				break;
> +		}
>   		if (signal_pending_state(state, current)) {
>   			raw_spin_lock_irq(&sem->wait_lock);
>   			if (waiter->task)
>