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

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.ÂFMODE_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.

With best regards,

David

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