Re: [PATCH v4 14/16] locking/rwsem: Guard against making count negative

[Waiman Long]

On 4/24/19 3:09 AM, Peter Zijlstra wrote:
> On Tue, Apr 23, 2019 at 03:12:16PM -0400, Waiman Long wrote:
>> That is true in general, but doing preempt_disable/enable across
>> function boundary is ugly and prone to further problems down the road.
> We do worse things in this code, and the thing Linus proposes is
> actually quite simple, something like so:
>
> ---
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -912,7 +904,7 @@ rwsem_down_read_slowpath(struct rw_semap
>  			raw_spin_unlock_irq(&sem->wait_lock);
>  			break;
>  		}
> -		schedule();
> +		schedule_preempt_disabled();
>  		lockevent_inc(rwsem_sleep_reader);
>  	}
>  
> @@ -1121,6 +1113,7 @@ static struct rw_semaphore *rwsem_downgr
>   */
>  inline void __down_read(struct rw_semaphore *sem)
>  {
> +	preempt_disable();
>  	if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
>  			&sem->count) & RWSEM_READ_FAILED_MASK)) {
>  		rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
> @@ -1129,10 +1122,12 @@ inline void __down_read(struct rw_semaph
>  	} else {
>  		rwsem_set_reader_owned(sem);
>  	}
> +	preempt_enable();
>  }
>  
>  static inline int __down_read_killable(struct rw_semaphore *sem)
>  {
> +	preempt_disable();
>  	if (unlikely(atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
>  			&sem->count) & RWSEM_READ_FAILED_MASK)) {
>  		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
> @@ -1142,6 +1137,7 @@ static inline int __down_read_killable(s
>  	} else {
>  		rwsem_set_reader_owned(sem);
>  	}
> +	preempt_enable();
>  	return 0;
>  }
>  

Making that change will help the slowpath to has less preemption points.
For an uncontended rwsem, this offers no real benefit. Adding
preempt_disable() is more complicated than I originally thought.

Maybe we are too paranoid about the possibility of a large number of
preemptions happening just at the right moment. If p is the probably of
a preemption in the middle of the inc-check-dec sequence, which I have
already moved as close to each other as possible. We are talking a
probability of p^32768. Since p will be really small, the compound
probability will be infinitesimally small.

So I would like to not do preemption now for the current patchset. We
can restart the discussion later on if there is a real concern that it
may actually happen. Please let me know if you still want to add
preempt_disable() for the read lock.

Cheers,
Longman