Return-Path: Received: from TYO201.gate.nec.co.jp ([210.143.35.51]:41233 "EHLO tyo201.gate.nec.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752078AbbJPBsA convert rfc822-to-8bit (ORCPT ); Thu, 15 Oct 2015 21:48:00 -0400 From: Kosuke Tatsukawa To: "J. Bruce Fields" CC: Trond Myklebust , Neil Brown , Anna Schumaker , Jeff Layton , "David S. Miller" , "linux-nfs@vger.kernel.org" , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v2] sunrpc: fix waitqueue_active without memory barrier in sunrpc Date: Fri, 16 Oct 2015 01:46:48 +0000 Message-ID: <17EC94B0A072C34B8DCF0D30AD16044A0287864B@BPXM09GP.gisp.nec.co.jp> In-Reply-To: <20151015205742.GB20155@fieldses.org> Content-Type: text/plain; charset="iso-2022-jp" MIME-Version: 1.0 Sender: linux-nfs-owner@vger.kernel.org List-ID: J. Bruce Fields wrote: > 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. sk_write_space will be called in sock_wfree() with UDP/IP each time kfree_skb() is called. With TCP/IP, sk_write_space is only called if SOCK_NOSPACE has been set. sk_data_ready will be called in both tcp_rcv_established() for TCP/IP and in sock_queue_rcv_skb() for UDP/IP. The latter lacks a memory barrier with sk_data_ready called right after __skb_queue_tail(). I think this hasn't caused any problems because sk_data_ready wasn't used. > --b. > > commit 0882cfeb39e0 > Author: J. Bruce Fields > 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 > > 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); > } --- Kosuke TATSUKAWA | 3rd IT Platform Department | IT Platform Division, NEC Corporation | tatsu@ab.jp.nec.com