Ocfs2 devel - Dec 2015 - [PATCH] ocfs2: dlm: fix recursive locking deadlock

On 12/14/2015 01:39 PM, Gang He wrote:
> Hello Junxiao,
> 
> From the initial description, the second lock_XYZ(PR) should be blocked,
since DLM have a fair queue  mechanism, otherwise, it looks to bring a write
lock starvation.
Should be blocked? No, that is a deadlock. I don't think this recursive
locking is common, so no need care starvation here.
> Second, this issue can be reproduced in old Linux kernels (e.g. 3.16.7-24)?
there should not be any regression issue?
Maybe just hard to reproduce, ocfs2 supports recursive locking.
> Finally, really do not like nested using lock, can we avoid this.
I didn't see a good reason why this should be avoided. Without this,
developer needs pay more attend to not involve recursive locking,
usually that is very hard before run a full test or a very detailed review.

Thanks,
Junxiao.
> 
> Thanks
> Gang
> 
>

Hi,

On Mon, Dec 14, 2015 at 02:03:17PM +0800, Junxiao Bi wrote:
> On 12/14/2015 01:39 PM, Gang He wrote:
> > Hello Junxiao,
> > 
> > From the initial description, the second lock_XYZ(PR) should be
blocked, since DLM have a fair queue  mechanism, otherwise, it looks to bring a
write lock starvation.
> Should be blocked? No, that is a deadlock. I don't think this recursive
> locking is common, so no need care starvation here.
"not common" is really good news. I think we should list recursive use
cases first and try to decrease that use before messing up
"__ocfs2_cluster_lock"
further. As for this patch,  cost is too high :/

Thanks,
Eric
> 
> > Second, this issue can be reproduced in old Linux kernels (e.g.
3.16.7-24)? there should not be any regression issue?
> Maybe just hard to reproduce, ocfs2 supports recursive locking.
> 
> > Finally, really do not like nested using lock, can we avoid this.
> I didn't see a good reason why this should be avoided. Without this,
> developer needs pay more attend to not involve recursive locking,
> usually that is very hard before run a full test or a very detailed review.
> 
> Thanks,
> Junxiao.
> > 
> > Thanks
> > Gang
> > 
> > 
> 
> 
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>

On Mon, Dec 14, 2015 at 02:03:17PM +0800, Junxiao Bi
wrote:
> > Second, this issue can be reproduced in old Linux kernels (e.g.
3.16.7-24)? there should not be any regression issue?
> Maybe just hard to reproduce, ocfs2 supports recursive locking.
In what sense? The DLM might but the FS should never be making use of such a
mechanism (it would be for userspace users).

We really can't add recursive locks without this getting rejected upstream.
There's a whole slew of reasons why we don't like those in the kernel.
	--Mark

--
Mark Fasheh