Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()

[J Bruce Fields <bfields@fieldses.org>]

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