Re: [Xen-devel] Deadlock in /proc/xen/xenbus watch+read on 3.17+ (maybe earlier)

[Vitaly Chernooky <vitalii.chernookyi@xxxxxxxxxxxxxxx>]

On Fri, Mar 20, 2015 at 6:04 AM, Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx> wrote:

On Thu, Mar 19, 2015 at 03:10:49PM +0200, Vitaly Chernooky wrote:
> David,
>
> On Thu, Mar 19, 2015 at 3:00 PM, David Vrabel <david.vrabel@xxxxxxxxxx>
> wrote:
>
> > On 19/03/15 12:10, Iurii Konovalenko wrote:
> > > Hi, guys!
> > >
> > > When I read, that I am not alone and that issue depends on kernel
> > > version, I decided to continue investigation.
> > > And I found why our threads locks on read/write operations.
> > > On Linux kernel 3.14+ syscalls of file read and write changed a bit:
> > > fdget() function was replaced by fdget_pos() - it is fdget() function
> > > plus additional position mutex lock for files with FMODE_ATOMIC_POS
> > > (files for inodes with S_IFREG flag set - regular nodes). As I thought
> > > our xen files are not regular and nonseekable, I hoped this flag is
> > > not set. But it is set. It is because our file system is created by
> > > function simple_fill_super(), and inside it this flag is hardly set:
> > > inode->i_mode = S_IFREG | files->mode;
> > > So, as a fast hack I made a patch: just made copy of this function for
> > > xen, which does not set this flag. It works for me. Could you please
> > > check if it works for you.
> >
> > I still can't get this to deadlock, but why not clear FMODE_ATOMIC_POS
> > in xenbus_file_open() ?
> >
>
> Because it is not the root of issue. FMODE_ATOMIC_POS is just one of
> results of bug. Iurii has fixed the root of issue but in suboptimal way. So
> we just need to have found optimal way.

I can just confirm that:
1. (unsurprisingly) the bug is still present in 4.0-rc4
2. both proposed fixes are effective

I'm not sure if removing S_IFREG completely is a good idea, I guess
there will be much more side effects...
What about another idea: xenbus_file_open uses nonseekable_open - this
looks like a good place to clear FMODE_ATOMIC_POS if present? It
doesn't make sense to get a lock for position on nonseekable file,
right?

The Open Group Base Specifications Issue 7ÂIEEE Std 1003.1, 2013 Edition requires from regular files to be seekable. But Linux kernel looks like Linus has own opinion on it :(((

With best regards,

Â

--
Best Regards,
Marek Marczykowski-GÃrecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

--

Vitaly Chernooky |ÂSenior Developer - Product Engineering and Development

GlobalLogic

P +380.44.4929695 ext.1136 M +380.63.6011802ÂS cvv_2k

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