Re: [Xen-devel] [PATCH] tools/hotplug: fix locking

[Zhigang Wang <zhigang.x.wang@xxxxxxxxxx>]

On 06/21/2012 07:49 AM, Ian Campbell wrote:
> On Thu, 2012-06-21 at 12:42 +0100, Ian Campbell wrote:
>> On Wed, 2012-06-20 at 18:53 +0100, Zhigang Wang wrote:
>>> On 06/20/2012 11:34 AM, Ian Campbell wrote:
>>>> On Wed, 2012-06-13 at 18:29 +0100, Zhigang Wang wrote:
>>>>> # HG changeset patch
>>>>> # User Zhigang Wang <zhigang.x.wang@xxxxxxxxxx>
>>>>> # Date 1339608534 14400
>>>>> # Node ID 650b03f412143c3057abbd0ae0e256e6b3fd2ba8
>>>>> # Parent  32034d1914a607d7b6f1f060352b4cac973600f8
>>>>> tools/hotplug: fix locking
>>>> A better title would be "tools/hotplug: Switch to locking with flock"
>>>> since "fix" is not very descriptive.
>>> Agree.
>>>> The commit message should also state why changing this scheme is
>>>> preferable to fixing the current one.
>>> I have two points:
>>>
>>> 1. No timeout: in the old one, if one process holding the lock more than 100
>>>    seconds, other processes could steal it.
>>> 2. No leftovers: if a process holding this lock is dead, it will close the 
>>> lock
>>>    file, so it will not affect other processes.
>>>
>>>> Is this flock tool widely available? Does it need to be added to the
>>>> dependencies in the README?
>>> It is widely distributed: it's part of the util-linux package. So I think no
>>> need to document it.
>>>>> The current locking implementation would allow two processes get the lock
>>>>> simultaneously:
>>>> [...snip shell trace...]
>>>>
>>>> Can you add a line or two of analysis to explain where in that shell
>>>> spew things are actually going wrong and why?
>>> I didn't spent much time on this complicated implement. But it fails for me 
>>> and
>>> also for others (from response).
>>>>> This patch is ported from Red Hat Enterprise Linux 5.8.
>>>> I think we need a Signed-off-by from the original patch author. (Unless
>>>> DCO clause b somehow applies?)
>>> I'm not sure how to handle this. There's no signed-off-by on the original 
>>> Red
>>> Hat patch. Could anyone in Red Hat help to get it signed off?
>> Perhaps Andrew Jones or Don Dutile can point us in the right direction
>> (both CCd) if you identify the precise source of the patch (i.e. the
>> name of the .src.rpm and the name of the patch within it)?
>>
>> I looked in kernel-2.6.18-308.8.2.el5.src.rpm (which seems fairly
>> recent) but maybe I'm looking in the wrong place.
> Nevermind, I found it in xen-3.0.3-135.el5_8.2.src.rpm (I'd have sworn
> that RH shipped kernel+xen in a single source package, oh well).
>
> %changelog says:
>         * Fri Sep 14 2007 Daniel P. Berrange <berrange@xxxxxxxxxx> - 
> 3.0.3-40.el5
>         - Rewrite locking in hotplug scripts to fix timeouts (rhbz #267861)
>
> Daniel CCd.
>
> I wonder if there were any followup fixes or changes, especially wrt to
> your point 1 above (the lack of a timeout now) requiring xend (or now
> libxl) changes?

We have another timeout in xend/xl: if device backend is not setup properly, the
device creation will fail.

Without any timeout for this locking, all backend uevent scripts will hang if
one goes wrong. Just stealing the lock upon timeoutis not a good idea: it may
cause error or damage. What about adding a long (10minutes) timeout? (by adding
-w/--wait/--timeout 600)

Thanks,

Zhigang


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