On Sun, Aug 03, 2014 at 01:03:10PM -0400, Trond Myklebust wrote:
> We're always _only_ waking up tasks from within the sp_threads list, so
> we know that they are enqueued and alive. The rq_wait waitqueue is just
> a distraction with extra atomic semantics.
>
> Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
> ---
> include/linux/sunrpc/svc.h | 1 -
> net/sunrpc/svc.c | 2 --
> net/sunrpc/svc_xprt.c | 32 +++++++++++++++++---------------
> 3 files changed, 17 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 1bc7cd05b22e..3ec769b65c7f 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -280,7 +280,6 @@ struct svc_rqst {
> int rq_splice_ok; /* turned off in gss privacy
> * to prevent encrypting page
> * cache pages */
> - wait_queue_head_t rq_wait; /* synchronization */
> struct task_struct *rq_task; /* service thread */
> };
>
> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
> index 5de6801cd924..dfb78c4f3031 100644
> --- a/net/sunrpc/svc.c
> +++ b/net/sunrpc/svc.c
> @@ -612,8 +612,6 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
> if (!rqstp)
> goto out_enomem;
>
> - init_waitqueue_head(&rqstp->rq_wait);
> -
> serv->sv_nrthreads++;
> spin_lock_bh(&pool->sp_lock);
> pool->sp_nrthreads++;
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index 2c30193c7a13..438e91c12851 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -348,8 +348,6 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
>
> cpu = get_cpu();
> pool = svc_pool_for_cpu(xprt->xpt_server, cpu);
> - put_cpu();
> -
> spin_lock_bh(&pool->sp_lock);
>
> if (!list_empty(&pool->sp_threads) &&
> @@ -382,10 +380,15 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
> printk(KERN_ERR
> "svc_xprt_enqueue: server %p, rq_xprt=%p!\n",
> rqstp, rqstp->rq_xprt);
> - rqstp->rq_xprt = xprt;
> + /* Note the order of the following 3 lines:
> + * We want to assign xprt to rqstp->rq_xprt only _after_
> + * we've woken up the process, so that we don't race with
> + * the lockless check in svc_get_next_xprt().
Sorry, I'm not following this: what exactly is the race?
--b.
> + */
> svc_xprt_get(xprt);
> + wake_up_process(rqstp->rq_task);
> + rqstp->rq_xprt = xprt;
> pool->sp_stats.threads_woken++;
> - wake_up(&rqstp->rq_wait);
> } else {
> dprintk("svc: transport %p put into queue\n", xprt);
> list_add_tail(&xprt->xpt_ready, &pool->sp_sockets);
> @@ -394,6 +397,7 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
>
> out_unlock:
> spin_unlock_bh(&pool->sp_lock);
> + put_cpu();
> }
>
> /*
> @@ -509,7 +513,7 @@ void svc_wake_up(struct svc_serv *serv)
> svc_thread_dequeue(pool, rqstp);
> rqstp->rq_xprt = NULL;
> */
> - wake_up(&rqstp->rq_wait);
> + wake_up_process(rqstp->rq_task);
> } else
> pool->sp_task_pending = 1;
> spin_unlock_bh(&pool->sp_lock);
> @@ -628,7 +632,6 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp, long timeout)
> {
> struct svc_xprt *xprt;
> struct svc_pool *pool = rqstp->rq_pool;
> - DECLARE_WAITQUEUE(wait, current);
> long time_left;
>
> /* Normally we will wait up to 5 seconds for any required
> @@ -654,15 +657,15 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp, long timeout)
> xprt = ERR_PTR(-EAGAIN);
> goto out;
> }
> - /* No data pending. Go to sleep */
> - svc_thread_enqueue(pool, rqstp);
> -
> /*
> * We have to be able to interrupt this wait
> * to bring down the daemons ...
> */
> set_current_state(TASK_INTERRUPTIBLE);
>
> + /* No data pending. Go to sleep */
> + svc_thread_enqueue(pool, rqstp);
> +
> /*
> * checking kthread_should_stop() here allows us to avoid
> * locking and signalling when stopping kthreads that call
> @@ -676,14 +679,13 @@ static struct svc_xprt *svc_get_next_xprt(struct svc_rqst *rqstp, long timeout)
> goto out;
> }
>
> - add_wait_queue(&rqstp->rq_wait, &wait);
> spin_unlock_bh(&pool->sp_lock);
>
> time_left = schedule_timeout(timeout);
> + __set_current_state(TASK_RUNNING);
>
> try_to_freeze();
>
> - remove_wait_queue(&rqstp->rq_wait, &wait);
> xprt = rqstp->rq_xprt;
> if (xprt != NULL)
> return xprt;
> @@ -786,10 +788,10 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> printk(KERN_ERR
> "svc_recv: service %p, transport not NULL!\n",
> rqstp);
> - if (waitqueue_active(&rqstp->rq_wait))
> - printk(KERN_ERR
> - "svc_recv: service %p, wait queue active!\n",
> - rqstp);
> +
> + /* Make sure the task pointer is set! */
> + if (WARN_ON_ONCE(!rqstp->rq_task))
> + rqstp->rq_task = current_task;
>
> err = svc_alloc_arg(rqstp);
> if (err)
> --
> 1.9.3
>
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