"J. Bruce Fields" <bfields@xxxxxxxxxxxx> writes:
> On Thu, Oct 15, 2015 at 11:44:20AM +0000, Kosuke Tatsukawa wrote:
>> Tatsukawa Kosuke wrote:
>> > J. Bruce Fields wrote:
>> >> Thanks for the detailed investigation.
>> >>
>> >> I think it would be worth adding a comment if that might help someone
>> >> having to reinvestigate this again some day.
>> >
>> > It would be nice, but I find it difficult to write a comment in the
>> > sunrpc layer why a memory barrier isn't necessary, using the knowledge
>> > of how nfsd uses it, and the current implementation of the network code.
>> >
>> > Personally, I would prefer removing the call to waitqueue_active() which
>> > would make the memory barrier totally unnecessary at the cost of a
>> > spin_lock + spin_unlock by unconditionally calling
>> > wake_up_interruptible.
>>
>> On second thought, the callbacks will be called frequently from the tcp
>> code, so it wouldn't be a good idea.
>
> So, I was even considering documenting it like this, if it's not
> overkill.
>
> Hmm... but if this is right, then we may as well ask why we're doing the
> wakeups at all. Might be educational to test the code with them
> removed.
>
> --b.
>
> commit 0882cfeb39e0
> Author: J. Bruce Fields <bfields@xxxxxxxxxx>
> Date: Thu Oct 15 16:53:41 2015 -0400
>
> svcrpc: document lack of some memory barriers.
>
> Kosuke Tatsukawa points out an odd lack of memory barriers in some sites
> here. I think the code's correct, but it's probably worth documenting.
>
> Reported-by: Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx>
>
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 856407fa085e..90480993ec4a 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -399,6 +399,25 @@ static int svc_sock_secure_port(struct svc_rqst *rqstp)
> return svc_port_is_privileged(svc_addr(rqstp));
> }
>
> +static void svc_no_smp_mb(void)
> +{
> + /*
> + * Kosuke Tatsukawa points out there should normally be an
> + * smp_mb() at the callsites of this function. (Either that or
> + * we could just drop the waitqueue_active() checks.)
> + *
> + * It appears they aren't currently necessary, though, basically
> + * because nfsd does non-blocking reads from these sockets, so
> + * the only places we wait on this waitqueue is in sendpage and
> + * sendmsg, which won't be waiting for wakeups on newly arrived
> + * data.
> + *
> + * Maybe we should add the memory barriers anyway, but these are
> + * hot paths so we'd need to be convinced there's no sigificant
> + * penalty.
> + */
> +}
> +
> /*
> * INET callback when data has been received on the socket.
> */
> @@ -414,7 +433,7 @@ static void svc_udp_data_ready(struct sock *sk)
> set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
> svc_xprt_enqueue(&svsk->sk_xprt);
> }
> - smp_mb();
> + svc_no_smp_mb();
> if (wq && waitqueue_active(wq))
> wake_up_interruptible(wq);
> }
> @@ -433,7 +452,7 @@ static void svc_write_space(struct sock *sk)
> svc_xprt_enqueue(&svsk->sk_xprt);
> }
>
> - smp_mb();
> + svc_no_smp_mb();
> if (wq && waitqueue_active(wq)) {
> dprintk("RPC svc_write_space: someone sleeping on %p\n",
> svsk);
> @@ -789,7 +808,7 @@ static void svc_tcp_listen_data_ready(struct sock *sk)
> }
>
> wq = sk_sleep(sk);
> - smp_mb();
> + svc_no_smp_mb();
> if (wq && waitqueue_active(wq))
> wake_up_interruptible_all(wq);
> }
> @@ -811,7 +830,7 @@ static void svc_tcp_state_change(struct sock *sk)
> set_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags);
> svc_xprt_enqueue(&svsk->sk_xprt);
> }
> - smp_mb();
> + svc_no_smp_mb();
> if (wq && waitqueue_active(wq))
> wake_up_interruptible_all(wq);
> }
> @@ -827,7 +846,7 @@ static void svc_tcp_data_ready(struct sock *sk)
> set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
> svc_xprt_enqueue(&svsk->sk_xprt);
> }
> - smp_mb();
> + svc_no_smp_mb();
> if (wq && waitqueue_active(wq))
> wake_up_interruptible(wq);
> }
> @@ -1599,7 +1618,7 @@ static void svc_sock_detach(struct svc_xprt *xprt)
> sk->sk_write_space = svsk->sk_owspace;
>
> wq = sk_sleep(sk);
> - smp_mb();
> + svc_no_smp_mb();
> if (wq && waitqueue_active(wq))
> wake_up_interruptible(wq);
> }
I would feel a lot more comfortable if you instead created:
static inline bool sunrpc_waitqueue_active(struct wait_queue_head *wq)
{
if (!wq)
return false;
/* long comment abot not needing a memory barrier */
return waitqueue_active(wq);
}
and then replace various "if (wq && waitqueue_active(wq))" calls with
if (sunrpc_waitqueue_active(wq))"
The comment seems readable and seems to make sense.
NeilBrown
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