On Wed 18-05-16 08:58:42, Dave Chinner wrote:
> On Tue, May 17, 2016 at 12:34:57PM -0700, Dan Williams wrote:
> > On Tue, May 17, 2016 at 11:29 AM, Jon Derrick
> > <jonathan.derrick@xxxxxxxxx> wrote:
> > > This patch fixes S_DAX bd_inode i_flag locking to conform to suggested
> >
> > A "fix" implies that its currently broken. I don't see how it is, not
> > until we add an ioctl method or other path that also tries to update
> > the flags outside of blkdev_get() context. So, I don't think this
> > patch stands on its own if you were intending it to be merged
> > separately.
> >
> > > locking rules. It is presumed that S_DAX is the only valid inode flag
> > > for a block device which subscribes to direct-access, and will restore
> > > any previously set flags if direct-access initialization fails.
> > >
> > > This reverts to i_flags behavior prior to
> > > bbab37ddc20bae4709bca8745c128c4f46fe63c5
> > > by allowing other bd_inode flags when DAX is disabled
> > >
> > > Signed-off-by: Jon Derrick <jonathan.derrick@xxxxxxxxx>
> > > ---
> > > fs/block_dev.c | 31 ++++++++++++++++++++++++++-----
> > > 1 file changed, 26 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > > index 20a2c02..d41e37f 100644
> > > --- a/fs/block_dev.c
> > > +++ b/fs/block_dev.c
> > > @@ -1159,6 +1159,20 @@ void bd_set_size(struct block_device *bdev, loff_t size)
> > > }
> > > EXPORT_SYMBOL(bd_set_size);
> > >
> > > +static void bd_add_dax(struct inode *inode)
> > > +{
> > > + inode_lock(inode);
> > > + inode->i_flags |= S_DAX;
> > > + inode_unlock(inode);
> > > +}
> > > +
> > > +static void bd_clear_dax(struct inode *inode)
> > > +{
> > > + inode_lock(inode);
> > > + inode->i_flags &= ~S_DAX;
> > > + inode_unlock(inode);
> > > +}
> >
> > Since this is inode generic should these helpers be prefixed "i_"
> > rather than "bd_"?
>
> Probably not, because in general filesystems are responsible for
> updating i_flags to reflect on-disk inode configuration and that's
> typically done under transaction contexts. e.g. through ioctl
> interfaces to set/clear flags that are stored on disk. As such,
> inode->i_flags is effectively protected by the filesystem specific
> locking heirarchy, not the generic inode_lock().
>
> e.g. have a look at XFS storing a persistent "DAX-enabled" flag in
> the inode, which can be set/cleared on individual inodes dynamically
> by FS_IOC_FSSETXATTR. The XFS i_flags update function assumes
> exclusive access to the field as it is called under locked
> transaction context. Similar code exists in ext4, btrfs, gfs2,
> etc....
So in case of ext4, we actually do use inode_lock() to protect against
racing IOC_SETFLAGS calls. i_flags is a strange mix and there are a few
(like S_DEAD or S_NOSEC) which are not persistent and those get set /
cleared by VFS. Some of those places (e.g. the clearing in
__vfs_setxattr_noperm()) are not really controlled by the filesystem
AFAICT. So when XFS doesn't use inode_lock() to protect i_flags updates it
can race with VFS on i_flags updates.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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