Date:	Thu, 06 Jun 2013 15:29:21 +0800
From:	vaughan <vaughan.cao@...cle.com>
To:	Jörn Engel <joern@...fs.org>
CC:	dgilbert@...erlog.com, JBottomley@...allels.com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sg: atomize check and set sdp->exclude in sg_open

于 2013年06月06日 15:19, vaughan 写道:
> 于 2013年06月05日 23:41, Jörn Engel 写道:
>> On Thu, 6 June 2013 00:16:45 +0800, vaughan wrote:
>>> 于 2013年06月05日 21:27, Jörn Engel 写道:
>>>> On Wed, 5 June 2013 17:18:33 +0800, vaughan wrote:
>>>>>
>>>>> Check and set sdp->exclude should be atomic when set in sg_open().
>>>>
>>>> The patch is line-wrapped.  More importantly, it doesn't seem to do
>>> It's shorter than the original line, so I just leave it like this...
>>
>> Sure.  What I meant by line-wrapped is that your mailer mangled the
>> patch.  Those two lines should have been one:
>>>>> -                       ((!sfds_list_empty(sdp) || get_exclude(sdp))
>>>>> ? 0 : set_exclude(sdp, 1)));
>>
>>>> what your description indicates it should do.  And lastly, does this
>>>> fix a bug, possibly even one you have a testcase for, or was it found
>>>> by code inspection?
>>> I found it by code inspection. A race condition may happen with the
>>> old code if two threads are both trying to open the same sg with
>>> O_EXCL simultaneously. It's possible that they both find fsds list
>>> is empty and get_exclude(sdp) returns 0, then they both call
>>> set_exclude() and break out from wait_event_interruptible and resume
>>> open. So it's necessary to check again with sg_open_exclusive_lock
>>> held to ensure only one can set sdp->exclude and return >0 to break
>>> out from wait_event loop.
>>
>> Makes sense.  And reading the code again, I have to wonder what monkey
>> came up with the get_exclude/set_exclude functions.
>>
>> Can I sucker you into a slightly larger cleanup?  I think the entire
>> "get_exclude(sdp)) ? 0 : set_exclude(sdp, 1)" should be simplified.
>> And once you add the try_set_exclude(), set_exclude will only ever do
>> clear_exclude, so you might as well rename and simplify that as well.
> I find my patch is not enough to avoid this race condition said above.
> Since sg_add_sfp() just do an add_to_list without check and wait_event
> check don't set a sign to announce a future add_to_list is on going, the
> time window between wait_event and sg_add_sfp gives others to open sg
> before the prechecked sg_add_sfp() called.
>
> The same case also happens when one shared and one exclude open occur
> simultaneously. If the shared open pass the precheck stage and ready to
> sg_add_sfp(). At this time another exclude open will also pass the check:
>    ((!sfds_list_empty(sdp) || get_exclude(sdp)) ? 0 :
> try_set_exclude(sdp)));
> Then, both open can succeed.
>
> I think the point is we separate the check&add routine and haven't set
> an sign to let others wait until the whole actions complete. I suppose
> we may change the steps a bit to avoid trouble like this. If we can
> malloc&initialize sfp at first, and then check&add sfp under the
> protection of sg_index_lock, everything seems to be quite simple.
We also should prevent sg_device change those parameters which are 
needed to copy to sfp during sfp initialization.

Regards,
Vaughan

>
>
> Regards,
> Vaughan
>
>>
>> Let no good deed go unpunished.
>>
>> Jörn
>>
>> --
>> It's just what we asked for, but not what we want!
>> -- anonymous
>>
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