From: J Bruce Fields <bfields@fieldses.org>
To: Trond Myklebust <trondmy@gmail.com>
Cc: linux-nfs@vger.kernel.org
Subject: Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()
Date: Thu, 3 Jan 2019 17:45:29 -0500 [thread overview]
Message-ID: <20190103224529.GA6907@fieldses.org> (raw)
In-Reply-To: <20190103141712.24381-1-trond.myklebust@hammerspace.com>
On Thu, Jan 03, 2019 at 09:17:12AM -0500, Trond Myklebust wrote:
> Use READ_ONCE() to tell the compiler to not optimse away the read of
> xprt->xpt_flags in svc_xprt_release_slot().
What exactly is the possible race here? And why is a READ_ONCE()
sufficient, as opposed to some memory barriers?
I may need to shut myself in a room with memory-barriers.txt, I'm pretty
hazy on these things.
--b.
>
> Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
> ---
> net/sunrpc/svc_xprt.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> index 51d36230b6e3..94d344325e22 100644
> --- a/net/sunrpc/svc_xprt.c
> +++ b/net/sunrpc/svc_xprt.c
> @@ -363,9 +363,11 @@ static void svc_xprt_release_slot(struct svc_rqst *rqstp)
>
> static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt)
> {
> - if (xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_CLOSE)))
> + unsigned long xpt_flags = READ_ONCE(xprt->xpt_flags);
> +
> + if (xpt_flags & (BIT(XPT_CONN) | BIT(XPT_CLOSE)))
> return true;
> - if (xprt->xpt_flags & ((1<<XPT_DATA)|(1<<XPT_DEFERRED))) {
> + if (xpt_flags & (BIT(XPT_DATA) | BIT(XPT_DEFERRED))) {
> if (xprt->xpt_ops->xpo_has_wspace(xprt) &&
> svc_xprt_slots_in_range(xprt))
> return true;
> --
> 2.20.1
next prev parent reply other threads:[~2019-01-03 22:45 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-03 14:17 [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot() Trond Myklebust
2019-01-03 22:45 ` J Bruce Fields [this message]
2019-01-03 23:40 ` Trond Myklebust
2019-01-04 17:39 ` bfields
2019-01-07 21:32 ` bfields
2019-01-07 22:06 ` Trond Myklebust
2019-01-08 15:01 ` bfields
2019-01-08 16:21 ` Trond Myklebust
2019-01-09 16:51 ` bfields
2019-01-09 17:41 ` Trond Myklebust
2019-01-11 21:12 ` bfields
2019-01-11 21:52 ` Chuck Lever
2019-01-11 21:54 ` Chuck Lever
2019-01-11 22:10 ` Bruce Fields
2019-01-11 22:27 ` Chuck Lever
2019-01-12 0:56 ` Bruce Fields
2019-01-14 17:24 ` Chuck Lever
2019-01-25 20:30 ` Bruce Fields
2019-01-25 21:32 ` Chuck Lever
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