Re: [PATCH] xfs_file_last_byte() needs to acquire ilock

[Eric Sandeen <sandeen@sandeen.net>]

Lachlan McIlroy wrote:
> We had some systems crash with this stack:
> 
> [<a00000010000cb20>] ia64_leave_kernel+0x0/0x280
> [<a00000021291ca00>] xfs_bmbt_get_startoff+0x0/0x20 [xfs]
> [<a0000002129080b0>] xfs_bmap_last_offset+0x210/0x280 [xfs]
> [<a00000021295b010>] xfs_file_last_byte+0x70/0x1a0 [xfs]
> [<a00000021295b200>] xfs_itruncate_start+0xc0/0x1a0 [xfs]
> [<a0000002129935f0>] xfs_inactive_free_eofblocks+0x290/0x460 [xfs]
> [<a000000212998fb0>] xfs_release+0x1b0/0x240 [xfs]
> [<a0000002129ad930>] xfs_file_release+0x70/0xa0 [xfs]
> [<a000000100162ea0>] __fput+0x1a0/0x420
> [<a000000100163160>] fput+0x40/0x60
> 
> The problem here is that xfs_file_last_byte() does not acquire the
> inode lock and can therefore race with another thread that is modifying
> the extext list.  While xfs_bmap_last_offset() is trying to lookup
> what was the last extent some extents were merged and the extent list
> shrunk so the index we lookup is now beyond the end of the extent list
> and potentially in a freed buffer.
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index e7ae08d..cf62d9d 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -1258,8 +1258,10 @@ xfs_file_last_byte(

        /*
         * Only check for blocks beyond the EOF if the extents have
         * been read in.  This eliminates the need for the inode lock,
         * and it also saves us from looking when it really isn't
>  	 * necessary.
>  	 */

I suppose that comment should be modified too, and maybe the commit log
should say why, exactly, it was wrong? :)

-Eric

>  	if (ip->i_df.if_flags & XFS_IFEXTENTS) {
> +		xfs_ilock(ip, XFS_ILOCK_SHARED);
>  		error = xfs_bmap_last_offset(NULL, ip, &last_block,
>  			XFS_DATA_FORK);
> +		xfs_iunlock(ip, XFS_ILOCK_SHARED);
>  		if (error) {
>  			last_block = 0;
>  		}
> 
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