From: "bfields@fieldses.org" <bfields@fieldses.org>
To: Trond Myklebust <trondmy@hammerspace.com>
Cc: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()
Date: Mon, 7 Jan 2019 16:32:18 -0500 [thread overview]
Message-ID: <20190107213218.GD7753@fieldses.org> (raw)
In-Reply-To: <20190104173912.GC11787@fieldses.org>
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.
next prev parent reply other threads:[~2019-01-07 21:32 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
2019-01-03 23:40 ` Trond Myklebust
2019-01-04 17:39 ` bfields
2019-01-07 21:32 ` bfields [this message]
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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