Re: [PATCH v2 00/13] rwsem fast-path write lock stealing

[Michel Lespinasse]

On Sat, Apr 27, 2013 at 02:15:52PM -0700, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> Subject: [PATCH] rwsem: check counter to avoid cmpxchg calls
> 
> This patch tries to reduce the amount of cmpxchg calls in the
> writer failed path by checking the counter value first before
> issuing the instruction. If ->count is not set to RWSEM_WAITING_BIAS
> then there is no point wasting a cmpxchg call.
> 
> Signed-off-by: Davidlohr Bueso <davidlohr.bueso@xxxxxx>
> ---
>  lib/rwsem.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 50fdd89..a79dc95 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -222,9 +222,10 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
>  			count = RWSEM_ACTIVE_WRITE_BIAS;
>  			if (!list_is_singular(&sem->wait_list))
>  				count += RWSEM_WAITING_BIAS;
> -			if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
> -							RWSEM_WAITING_BIAS)
> -				break;
> +			if ((*(volatile long *)&(sem)->count) == RWSEM_WAITING_BIAS)
> +				if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
> +				    RWSEM_WAITING_BIAS)
> +					break;
>  		}
>  
>  		raw_spin_unlock_irq(&sem->wait_lock);

Quite impressed that this makes as much of a difference - this code block
already checks that the active count was 0 (which implies the sem->count
must be RWSEM_WAITING_BIAS, since the writer thread is known to be waiting)
not long before. But I suppose it helps due to the case where someone else
steals the lock while we're trying to acquire sem->wait_lock.

Regarding style: I would prefer if the sem->count read didn't use the
volatile cast. The compiler will already be forced to reload the value
due to the barriers in set_task_state() and raw_spin_lock_irq(). And if
you absolutely need to have that volatile, I would prefer you use
ACCESS_ONCE() rather than the hand written equivalent.

I'm going to try pushing this to Linus tomorrow; I feel I shouldn't change
your patch without putting it through tests again; are you OK with sending
this as a followup to the series rather than me including it ?

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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