Re: [PATCH] sg: atomize check and set sdp->exclude in sg_open

[vaughan]

ä 2013å06æ06æ 15:19, vaughan åé:
> ä 2013å06æ05æ 23:41, JÃrn Engel åé:
> > On Thu, 6 June 2013 00:16:45 +0800, vaughan wrote:
> > > ä 2013å06æ05æ 21:27, JÃrn Engel åé:
> > > > 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.
> >
> > JÃrn
> >
> > --
> > It's just what we asked for, but not what we want!
> > -- anonymous
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