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

bfields
2019-01-07T21:32:19Z

On Fri, Jan 04, 2019 at 12:39:12PM -0500, bfields@fieldses.org wrote:
> I wonder if there's a race here independent of that change:
>
> svc_xprt_enqueue() callers all do something like:
>
> 1. change some condition
> 2. call svc_xprt_enqueue() to check whether the xprt should
> now be enqueued.
>
> where the conditions are settings of the xpt_flags, or socket wspace, or
> xpt_nr_rqsts.
>
> In theory if we miss some concurrent change we're OK because whoever's
> making that change will then also call svc_xprt_enqueue. But that's not
> enough; e.g.:
>
> task 1 task 2
> ------ ------
> set XPT_DATA
> atomic_dec(xpt_nr_rqsts)
>
> check XPT_DATA && check xpt_nr_rqsts
>
> check XPT_DATA && check xpt_nr_rqsts
>
> If the tasks only see their local changes, then neither see both
> conditions true, so the socket doesn't get enqueued. (And a request
> that was ready to be processed will sit around until someone else comes
> calls svc_xprt_enqueue() on that xprt.)

So maybe we actually need

static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt)
{
+ mb();
if (xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_CLOSE)))
return true;
if (xprt->xpt_flags & ((1<<XPT_DATA)|(1<<XPT_DEFERRED))) {

Then whichever memory barrier executes second guarantees that the
following check sees the result of both the XPT_DATA and xpt_nr_rqsts
changes. I think....

--b.