linux-kernel - Re: [V9fs-developer] Hang triggered by udev coldplug, looks like a race

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Message-ID: <20151208023331.GJ20997@ZenIV.linux.org.uk>
Date:	Tue, 8 Dec 2015 02:33:31 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dominique Martinet <dominique.martinet@....fr>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [V9fs-developer] Hang triggered by udev coldplug, looks like a
 race

On Mon, Dec 07, 2015 at 05:59:41PM -0800, Andy Lutomirski wrote:

> If I dump all task states (see attached typescript), I see a bunch of
> things blocked in 9p rpc.  This makes me think it could be a QEMU bug,
> not a kernel bug.

Maybe, maybe not - I'd suggest dumping the 9p traffic and checking the
request and response IDs; at least one bug in that area got fixed
(see commit a84b69 for gory details).

Really, check if something gets screwed with IDs.  They should be unique
among outstanding requests and never reused until either a response
with the matching ID arrives, *or* TFLUSH on that ID is sent and reply
has arrived.   If those rules get buggered, it's very easy to get hangs
like that.
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