netdev - Re: [PATCH v2] sunrpc: fix waitqueue_active without memory barrier in sunrpc

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Date:	Fri, 09 Oct 2015 16:56:06 +1100
From:	Neil Brown <nfbrown@...ell.com>
To:	Kosuke Tatsukawa <tatsu@...jp.nec.com>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	"David S. Miller" <davem@...emloft.net>
Cc:	"linux-nfs\@vger.kernel.org" <linux-nfs@...r.kernel.org>,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sunrpc: fix waitqueue_active without memory barrier in sunrpc

Kosuke Tatsukawa <tatsu@...jp.nec.com> writes:

> There are several places in net/sunrpc/svcsock.c which calls
> waitqueue_active() without calling a memory barrier.  Add a memory
> barrier just as in wq_has_sleeper().
>
> I found this issue when I was looking through the linux source code
> for places calling waitqueue_active() before wake_up*(), but without
> preceding memory barriers, after sending a patch to fix a similar
> issue in drivers/tty/n_tty.c  (Details about the original issue can be
> found here: https://lkml.org/lkml/2015/9/28/849).

hi,
this feels like the wrong approach to the problem.  It requires extra
'smb_mb's to be spread around which are hard to understand as easy to
forget.

A quick look seems to suggest that (nearly) every waitqueue_active()
will need an smb_mb.  Could we just put the smb_mb() inside
waitqueue_active()??

Thanks,
NeilBrown


>
> Signed-off-by: Kosuke Tatsukawa <tatsu@...jp.nec.com>
> ---
> v2:
>   - Fixed compiler warnings caused by type mismatch
> v1:
>   - https://lkml.org/lkml/2015/10/8/993
> ---
>  net/sunrpc/svcsock.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 0c81202..ec19444 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -414,6 +414,7 @@ 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();
>  	if (wq && waitqueue_active(wq))
>  		wake_up_interruptible(wq);
>  }

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