linux-kernel - Re: [PATCH V4 09/13] fs/xfs: Add write aops lock to xfs layer

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Date:   Tue, 25 Feb 2020 13:12:28 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V4 09/13] fs/xfs: Add write aops lock to xfs layer

On Tue, Feb 25, 2020 at 09:32:45AM +1100, Dave Chinner wrote:
> On Mon, Feb 24, 2020 at 11:57:36AM -0800, Ira Weiny wrote:
> > On Mon, Feb 24, 2020 at 11:34:55AM +1100, Dave Chinner wrote:
> > > On Thu, Feb 20, 2020 at 04:41:30PM -0800, ira.weiny@...el.com wrote:
> > > > From: Ira Weiny <ira.weiny@...el.com>
> > > > 
> > 
> > [snip]
> > 
> > > > 
> > > > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > > > index 35df324875db..5b014c428f0f 100644
> > > > --- a/fs/xfs/xfs_inode.c
> > > > +++ b/fs/xfs/xfs_inode.c
> > > > @@ -142,12 +142,12 @@ xfs_ilock_attr_map_shared(
> > > >   *
> > > >   * Basic locking order:
> > > >   *
> > > > - * i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
> > > > + * s_dax_sem -> i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
> > > >   *
> > > >   * mmap_sem locking order:
> > > >   *
> > > >   * i_rwsem -> page lock -> mmap_sem
> > > > - * mmap_sem -> i_mmap_lock -> page_lock
> > > > + * s_dax_sem -> mmap_sem -> i_mmap_lock -> page_lock
> > > >   *
> > > >   * The difference in mmap_sem locking order mean that we cannot hold the
> > > >   * i_mmap_lock over syscall based read(2)/write(2) based IO. These IO paths can
> > > > @@ -182,6 +182,9 @@ xfs_ilock(
> > > >  	       (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL));
> > > >  	ASSERT((lock_flags & ~(XFS_LOCK_MASK | XFS_LOCK_SUBCLASS_MASK)) == 0);
> > > >  
> > > > +	if (lock_flags & XFS_DAX_EXCL)
> > > > +		inode_aops_down_write(VFS_I(ip));
> > > 
> > > I largely don't see the point of adding this to xfs_ilock/iunlock.
> > > 
> > > It's only got one caller, so I don't see much point in adding it to
> > > an interface that has over a hundred other call sites that don't
> > > need or use this lock. just open code it where it is needed in the
> > > ioctl code.
> > 
> > I know it seems overkill but if we don't do this we need to code a flag to be
> > returned from xfs_ioctl_setattr_dax_invalidate().  This flag is then used in
> > xfs_ioctl_setattr_get_trans() to create the transaction log item which can then
> > be properly used to unlock the lock in xfs_inode_item_release()
> > 
> > I don't know of a cleaner way to communicate to xfs_inode_item_release() to
> > unlock i_aops_sem after the transaction is complete.
> 
> We manually unlock inodes after transactions in many cases -
> anywhere we do a rolling transaction, the inode locks do not get
> released by the transaction. Hence for a one-off case like this it
> doesn't really make sense to push all this infrastructure into the
> transaction subsystem. Especially as we can manually lock before and
> unlock after the transaction context without any real complexity.

So does xfs_trans_commit() operate synchronously?

I want to understand this better because I have fought with a lot of ABBA
issues with these locks.  So...  can I hold the lock until after
xfs_trans_commit() and safely unlock it there... because the XFS_MMAPLOCK_EXCL,
XFS_IOLOCK_EXCL, and XFS_ILOCK_EXCL will be released at that point?  Thus
preserving the following lock order.

...
 * Basic locking order:
 *
 * i_aops_sem -> i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
 *
...

Thanks for the review!
Ira

> 
> This also means that we can, if necessary, do aops manipulation work
> /after/ the transaction that changes on-disk state completes and we
> still hold the aops reference exclusively. While we don't do that
> now, I think it is worthwhile keeping our options open here....
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com