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

Lachlan McIlroy
2009-04-24T03:46:54Z

----- "Eric Sandeen" <sandeen@sandeen.net> wrote:

> 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? :)
Ha, I didn't even read the comment! It's still kind of correct in
that we wont have to get the inode lock if the extents have not been
read in.

>
> -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;
> > }
> >
> > _______________________________________________
> > xfs mailing list
> > xfs@oss.sgi.com
> > http://oss.sgi.com/mailman/listinfo/xfs
> >