Re: [PATCH 01/12] SUNRPC: make rqst_should_sleep() idempotent()

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2023-07-31 at 16:48 +1000, NeilBrown wrote:
> Based on its name you would think that rqst_should_sleep() would be
> read-only, not changing anything.  But it fact it will clear
> SP_TASK_PENDING if that was set.  This is surprising, and it blurs the
> line between "check for work to do" and "dequeue work to do".
> 
> So change the "test_and_clear" to simple "test" and clear the bit once
> the thread has decided to wake up and return to the caller.
> 
> With this, it makes sense to *always* set SP_TASK_PENDING when asked,
> rather than only to set it if no thread could be woken up.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  net/sunrpc/svc_xprt.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index cd92cb54132d..380fb3caea4c 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -581,8 +581,8 @@ void svc_wake_up(struct svc_serv *serv)
>  {
>  	struct svc_pool *pool = &serv->sv_pools[0];
>  
> -	if (!svc_pool_wake_idle_thread(serv, pool))
> -		set_bit(SP_TASK_PENDING, &pool->sp_flags);
> +	set_bit(SP_TASK_PENDING, &pool->sp_flags);
> +	svc_pool_wake_idle_thread(serv, pool);
>  }
>  EXPORT_SYMBOL_GPL(svc_wake_up);
>  
> @@ -704,7 +704,7 @@ rqst_should_sleep(struct svc_rqst *rqstp)
>  	struct svc_pool		*pool = rqstp->rq_pool;
>  
>  	/* did someone call svc_wake_up? */
> -	if (test_and_clear_bit(SP_TASK_PENDING, &pool->sp_flags))
> +	if (test_bit(SP_TASK_PENDING, &pool->sp_flags))
>  		return false;
>  
>  	/* was a socket queued? */
> @@ -750,6 +750,7 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp)
>  
>  	set_bit(RQ_BUSY, &rqstp->rq_flags);
>  	smp_mb__after_atomic();
> +	clear_bit(SP_TASK_PENDING, &pool->sp_flags);

Took me a few mins to decide that splitting up the test_and_clear_bit
didn't open a ToC/ToU race. I think we're saved by the fact that only
nfsd thread itself clears the bit, so we're guaranteed not to race with
another clear (whew).
 
>  	rqstp->rq_xprt = svc_xprt_dequeue(pool);
>  	if (rqstp->rq_xprt) {
>  		trace_svc_pool_awoken(rqstp);
> @@ -761,6 +762,7 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp)
>  	percpu_counter_inc(&pool->sp_threads_no_work);
>  	return NULL;
>  out_found:
> +	clear_bit(SP_TASK_PENDING, &pool->sp_flags);
>  	/* Normally we will wait up to 5 seconds for any required
>  	 * cache information to be provided.
>  	 */

Reviewed-by: Jeff Layton <jlayton@kernel.org>