Re: [RFC PATCH-tip v4 02/10] locking/rwsem: Stop active read lock ASAP

[Dave Chinner]

On Mon, Oct 10, 2016 at 02:34:34AM -0700, Christoph Hellwig wrote:
> On Mon, Oct 10, 2016 at 05:07:45PM +1100, Dave Chinner wrote:
> > > > *However*, the DAX IO path locking in XFS  has changed in 4.9-rc1 to
> > > > match the buffered IO single writer POSIX semantics - the test is a
> > > > bad test based on the fact it exercised a path that is under heavy
> > > > development and so can't be used as a regression test across
> > > > multiple kernels.
> > > 
> > > That being said - I wonder if we should allow the shared lock on DAX
> > > files IFF the user is specifying O_DIRECT in the open mode..
> > 
> > It should do - if it doesn't then we screwed up the IO path
> > selection logic in XFS and we'll need to fix it.
> 
> Depends on your defintion of "we".  The DAX code has always abused the
> direct I/O path, and that abuse is ingrained in the VFS path in a way that
> we can't easily undo it in XFS, e.g. take a look at io_is_direct and
> iocb_flags in include/linux/fs.h, which ensure that DAX I/O will always
> appear as IOCB_DIRECT to the fs. 

Um, I seem to have completely missed that change - when did that
happen and why?

Oh, it was part of the misguided "enable DAX on block devices"
changes - it was supposed to fix a problem with block device access
using buffered IO instead of DAX (commit 65f87ee71852 "fs, block:
force direct-I/O for dax-enabled block devices")).  The original
block device DAX code was reverted soon after this because it was
badly flawed, but this change was not removed at the same time
(probably because it was forgotton about).

Hence I'd suggest that DAX check in io_is_direct() should be removed
ASAP; the filesystems don't need it as they check the inode DAX
state directly, and the code it "fixed" is no longer in the tree.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx