On Mon, Jul 23, 2018 at 09:49:41AM +0200, Christoph Hellwig wrote: > On Sun, Jul 22, 2018 at 09:23:11AM +1000, Dave Chinner wrote: > > On Tue, Jul 17, 2018 at 04:24:05PM -0700, Christoph Hellwig wrote: > > > @@ -333,12 +335,15 @@ xfs_map_blocks( > > > * COW fork blocks can overlap data fork blocks even if the blocks > > > * aren't shared. COW I/O always takes precedent, so we must always > > > * check for overlap on reflink inodes unless the mapping is already a > > > - * COW one. > > > + * COW one, or the COW fork hasn't changed from the last time we looked > > > + * at it. > > > */ > > > imap_valid = offset_fsb >= wpc->imap.br_startoff && > > > offset_fsb < wpc->imap.br_startoff + wpc->imap.br_blockcount; > > > if (imap_valid && > > > - (!xfs_inode_has_cow_data(ip) || wpc->io_type == XFS_IO_COW)) > > > + (!xfs_inode_has_cow_data(ip) || > > > + wpc->io_type == XFS_IO_COW || > > > + wpc->cow_seq == ip->i_cowfp->if_seq)) > > > return 0; > > > > This seqno check is still racy against concurrent extent tree > > modification. We aren't holding the ILOCK here at all, the sequence > > numbers aren't atomic and there are no memory barriers to serialise > > cross-cpu load/store operations... > > mutex_unlock contains a barrier, and the sequence is a single register > read. There is nothing holding a lock here would help us with. Which mutex_unlock is that? I took a second look at the i_cowfp access and realized that we don't take ILOCK_SHARED until after the comparison. Writeback doesn't take any of the other inode locks AFAIK... so either there's some locking subtlety here that ought to be explained in a comment, or is this a bug waiting to happen? --D > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
