On Tue 15-04-14 15:26:42, Ted Tso wrote: > On Tue, Apr 15, 2014 at 06:26:27PM +0200, Jan Kara wrote: > > On Sat 12-04-14 09:45:27, Ted Tso wrote: > > > The function ext4_update_i_disksize() is used in only one place, in > > > the function mpage_map_and_submit_extent(). Move there to simplify > > > the code paths, and also move the call to ext4_mark_inode_dirty() into > > > the i_data_sem's critical region, to be consistent with all of the > > > other places where we update i_disksize. That way, we also keep the > > > raw_inode's i_disksize protected. > > > > > > Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx> > > > Cc: stable@xxxxxxxxxxxxxxx > > I agree that it makes sense to have all the places consistent and protect > > raw disk inode i_disksize with i_data_sem. OTOH I don't see a way how this > > can cause any real harm (but I guess you expect there might be something as > > you CCed stable), so can you explain it please? > > This was the case I was worried about: > > CPU #1 CPU #2 > > 1. down_write(&i_data_sem) > 2. Modify i_disk_size > 4. up_write(&i_data_sem) > 5. down_write(&i_data_sem) > 6. Modify i_disk_size > 7. Copy i_disk_size to on-disk inode > 8. up_write(&i_data_sem) > 9. Copy i_disk_size to on-disk inode > > > It's the standard data race; it might not be a problem on Intel CPU's, > but in general, cpu #1 might still have a stale copy of i_disk_size in > its cache, and hence it might copying the old, outdated value into the > on-disk inode. Yes, that could be a problem even on Intel CPU - not because of cache coherency but because old i_disk_size value might be speculatively preloaded before CPU#2 updates its value. So feel free to add: Reviewed-by: Jan Kara <jack@xxxxxxx> Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
