On Tue 14-04-20 14:37:21, Jeff Layton wrote: > On Tue, 2020-04-14 at 18:26 +0200, Jan Kara wrote: > > On Tue 14-04-20 08:04:09, Jeff Layton wrote: > > > From: Jeff Layton <jlayton@xxxxxxxxxx> > > > > > > When syncing out a block device (a'la __sync_blockdev), any error > > > encountered will only be recorded in the bd_inode's mapping. When the > > > blockdev contains a filesystem however, we'd like to also record the > > > error in the super_block that's stored there. > > > > > > Make mark_buffer_write_io_error also record the error in the > > > corresponding super_block when a writeback error occurs and the block > > > device contains a mounted superblock. > > > > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx> > > > > The patch looks good to me. I'd just note that bh->b_bdev->bd_super > > dereference is safe only because we will flush all dirty data when > > unmounting a filesystem which is somewhat tricky. Maybe that warrants a > > comment? Otherwise feel free to add: > > > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > Oh, hmm...now that I look again, I'm not sure this is actually safe. > > bh->b_bdev gets cleared out as we discard the buffer, so I don't think > that could end up getting zeroed while we're still using it. Correct. > The bd_super pointer gets zeroed out in kill_block_super, and after that > point it calls sync_blockdev(). Could writeback error processing race > with kill_block_super such that bd_inode gets set to NULL after we test > it but before we dereference it? Yeah, you're right. But you can avoid the race with READ_ONCE(bh->b_bdev->bd_super) and a big fat comment explaining why it is safe... :) Or you could be less daring and put rcu protection there because superblocks are RCU freed... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
