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RE: [PATCH] xfs: reset the i_iolock lock class in the reclaim path

To: "Christoph Hellwig" <hch@xxxxxxxxxxxxx>
Subject: RE: [PATCH] xfs: reset the i_iolock lock class in the reclaim path
From: "Alex Elder" <aelder@xxxxxxx>
Date: Mon, 2 Nov 2009 15:54:02 -0600
Cc: "Peter Zijlstra" <a.p.zijlstra@xxxxxxxxx>, <xfs@xxxxxxxxxxx>
In-reply-to: <20091019040526.GC21115@xxxxxxxxxxxxx>
Thread-index: AcpQcg52Kz2JU195SYiNJj0dLdp9SwLkXrjw
Thread-topic: [PATCH] xfs: reset the i_iolock lock class in the reclaim path
Christoph Hellwig wrote:
> The iolock is used for protecting reads, writes and block truncates against
> each other.  We have two classes of callers, the first one is induced by
> a file operation and requires a reference to the inode be held and not
> dropped after the operation is done:
> 
>  - xfs_vm_vmap, xfs_vn_fallocate, xfs_read, xfs_write, xfs_splice_read,
>    xfs_splice_write and xfs_setattr are all implementations of VFS methods
>    that require a live inode
>  - xfs_getbmap and xfs_swap_extents are ioctl subcommand for which the
>    same is true
>  - xfs_truncate_file is only called on quota inodes just returned from 
> xfs_iget
>  - xfs_sync_inode_data does the lock just after an igrab()
>  - xfs_filestream_associate and xfs_filestream_new_ag take the iolock on the
>    parent inode of an inode which by VFS rules must be referenced
> 
> And we have various calls to truncate blocks past EOF or the whole file when
> dropping the last reference to an inode.   Unfortunately lockdep complains
> when we do memory allocations that can recurse into the filesystem in the
> first class because the second class happens to take the same lock.  To avoid
> this re-init the iolock in the beginning of xfs_fs_clear_inode to get
> a new lock class.
> 
> 
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Looks good.

The comment in xfs_fs_clear_inode() is very informative, but
it may be emphasizing a lot of detail that doesn't really
help the reader at this spot in the code.  What about wording
something more like this:
    The iolock is used by the file system to coordinate
    reads, writes, and block truncates.  Up to this point
    the lock protected concurrent accesses by users of
    the inode.  But from here forward we're doing some final
    processing of the inode because we're done with it,
    and although we reuse the iolock for protection it is
    really a distinct lock class (in the lockdep sense) from
    before.  To keep lockdep happy (and basically indicate
    what we are doing), we explicitly re-init the iolock here.

Reviewed-by: Alex Elder <aelder@xxxxxxx>


> Index: xfs/fs/xfs/linux-2.6/xfs_super.c
> ===================================================================
> --- xfs.orig/fs/xfs/linux-2.6/xfs_super.c     2009-10-14 17:24:31.356278624 
> +0200
> +++ xfs/fs/xfs/linux-2.6/xfs_super.c  2009-10-19 06:03:05.771006625 +0200
> @@ -999,7 +999,6 @@ xfs_fs_inode_init_once(
> 
>       mrlock_init(&ip->i_lock, MRLOCK_ALLOW_EQUAL_PRI|MRLOCK_BARRIER,
>                    "xfsino", ip->i_ino);
> -     mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
>  }
> 
>  /*
> @@ -1101,6 +1100,22 @@ xfs_fs_clear_inode(
>       XFS_STATS_INC(vn_remove);
>       XFS_STATS_DEC(vn_active);
> 
> +     /*
> +      * The iolock is used for protecting reads, writes and block truncates
> +      * against each other.  We have two classes of callers, the first one
> +      * is induced by a file operation and requires a reference to the
> +      * inode be held and not dropped after the operation is done, and
> +      * second we have various calls to truncate blocks past EOF or for the
> +      * whole file when dropping the last reference to an inode.
> +      * Unfortunately lockdep complains when we do memory allocations that
> +      * can recurse into the filesystem in the first class because the
> +      * second class happens to take the same lock.  To avoid this
> +      * reinitialize the iolock in the beginning of xfs_fs_clear_inode to
> +      * get a new lock class.
> +      */
> +     ASSERT(!rwsem_is_locked(&ip->i_iolock.mr_lock));
> +     mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
> +
>       xfs_inactive(ip);
>  }
> 
> Index: xfs/fs/xfs/xfs_iget.c
> ===================================================================
> --- xfs.orig/fs/xfs/xfs_iget.c        2009-10-14 17:25:26.733004131 +0200
> +++ xfs/fs/xfs/xfs_iget.c     2009-10-14 17:28:26.272274357 +0200
> @@ -73,6 +73,9 @@ xfs_inode_alloc(
>       ASSERT(atomic_read(&ip->i_pincount) == 0);
>       ASSERT(!spin_is_locked(&ip->i_flags_lock));
>       ASSERT(completion_done(&ip->i_flush));
> +     ASSERT(!rwsem_is_locked(&ip->i_iolock.mr_lock));
> +
> +     mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
> 
>       /* initialise the xfs inode */
>       ip->i_ino = ino;
> 
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