Re: Correctness of inode_dio_end in generic DIO code

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 20, 2018 at 10:59:46AM +0200, Nikolay Borisov wrote:
> Hello,
> 
> Currently the generic DIO code calls inode_dio_begin/inode_dio_end if
> DIO_SKIP_DIO_COUNT is not set.

DIO_SKIP_DIO_COUNT is not used by anyone. It's dead code, so
probably should be removed.

> However, te generic ode doesn't really
> know if there is a lock synchronizing all the various inode_dio_*
> operations. As per inode_dio_wait comment :
> 
> Must be called under a lock that serializes taking new references to
> i_dio_count, usually by inode->i_mutex.

Yup. DIO_LOCKING fileystems all use inode->i_rwsem. Filesystems that
don't use DIO_LOCKING need to provide their own locking.

Locking for DIO submission is all explained in the comment above
do_blockdev_direct_IO(). inode_dio_begin() is covered by the IO
submission locking scheme...

> So is it at all correct to increment i_dio_count in generic dio code
> without imposing strict locking requirement?

Most filesystems call blockdev_direct_IO() which sets DIO_LOCKING.

> Currently, most major
> filesystems (Ext4/xfs/btrfs) do modify i_dio_count under their own
> locks.

Sort of.

Both btrfs and ext4 use DIO_LOCKING directly, except in certain
configs ext4 doesn't do any locking at all.

XFS uses it's "own locking", but that's actually inode->i_rwsem now,
Also, XFS also uses iomap_dio_rw(), which is a new, more efficient
direct IO code path with a separate call to inode_dio_begin() under
"caller must lock IO submission" rules....

> Perhaps it's best if i_dio_count modification are removed from
> the generic code, what do people think about that?

IMO, it's in the correct spot - it's always accounted and called
under the correct IO submission locks where it is. Removing it from
the generic code will simply introduce bugs in new/lesser travelled
filesystems where they forget to call it or call it incorrectly.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com