On Fri, Mar 20, 2015 at 6:04 AM, Marek Marczykowski-Górecki < marma...@invisiblethingslab.com> 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.vra...@citrix.com> > > 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 7 IEEE 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, > -- > Best Regards, > Marek Marczykowski-Górecki > 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 | Senior Developer - Product Engineering and Development* GlobalLogic P *+380.44.4929695 ext.1136* M *+380.63.6011802* S cvv_2k www.globallogic.com http://www.globallogic.com/email_disclaimer.txt
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