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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.

  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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