Re: [PATCH 04/11] ext4: completed_io locking cleanup V4

[Jan Kara <jack@suse.cz>]

On Tue 02-10-12 14:57:22, Dmitry Monakhov wrote:
> On Tue, 2 Oct 2012 12:31:41 +0200, Jan Kara <jack@suse.cz> wrote:
> > On Tue 02-10-12 11:16:38, Dmitry Monakhov wrote:
> > > > > +	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> > > > > +	while (!list_empty(&complete)) {
> > > > > +		io = list_entry(complete.next, ext4_io_end_t, list);
> > > > > +		io->flag &= ~EXT4_IO_END_UNWRITTEN;
> > > > > +		/* end_io context can not be destroyed now because it still
> > > > > +		 * used by queued worker. Worker thread will destroy it later */
> > > > > +		if (io->flag & EXT4_IO_END_QUEUED)
> > > > > +			list_del_init(&io->list);
> > > > > +		else
> > > > > +			list_move(&io->list, &to_free);
> > > > > +	}
> > > > > +	/* If we are called from worker context, it is time to clear queued
> > > > > +	 * flag, and destroy it's end_io if it was converted already */
> > > > > +	if (work_io) {
> > > > > +		work_io->flag &= ~EXT4_IO_END_QUEUED;
> > > > > +		if (!(work_io->flag & EXT4_IO_END_UNWRITTEN))
> > > > > +			list_add_tail(&work_io->list, &to_free);
> > > > >  	}
> > > > > -	list_del_init(&io->list);
> > > > >  	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> > > > > -	(void) ext4_end_io_nolock(io);
> > > > > -	mutex_unlock(&inode->i_mutex);
> > > > > -free:
> > > > > -	ext4_free_io_end(io);
> > > > > +
> > > > > +	while (!list_empty(&to_free)) {
> > > > > +		io = list_entry(to_free.next, ext4_io_end_t, list);
> > > > > +		list_del_init(&io->list);
> > > > > +		ext4_free_io_end(io);
> > > > > +	}
> > > > > +	return ret;
> > > > > +}
> > > > > +
> > > > > +/*
> > > > > + * work on completed aio dio IO, to convert unwritten extents to extents
> > > > > + */
> > > > > +static void ext4_end_io_work(struct work_struct *work)
> > > > > +{
> > > > > +	ext4_io_end_t *io = container_of(work, ext4_io_end_t, work);
> > > > > +	ext4_do_flush_completed_IO(io->inode, io);
> > > > > +}
> > > > > +
> > > > > +int ext4_flush_completed_IO(struct inode *inode)
> > > > > +{
> > > > > +	return ext4_do_flush_completed_IO(inode, NULL);
> > > > >  }
> > > >   Also it seems that when ext4_flush_completed_IO() is called, workqueue
> > > > can have several IO structures queued in its local lists thus we miss them
> > > > here and don't properly wait for all conversions?
> > > No it is not. Because list drained atomically, and 
> > > add_complete_io will queue work only if list is empty.
> > > 
> > > Race between conversion and dequeue-process is not possible because
> > > we hold lock during entire walk of complete_list, so from external
> > > point of view we mark list as conversed(clear unwritten flag)
> > > happens atomically. I've drawn all possible situations and race not
> > > happen. If you know any please let me know.
> >   I guess I'm missing something obvious. So let's go step by step:
> > 1) ext4_flush_completed_IO() must make sure there is no outstanding
> >    conversion for the inode.
> > 2) Now assume we have non-empty i_completed_io_list - thus work is queued.
> > 3) The following situation seems to be possible:
> > 
> > CPU1					CPU2
> > (worker thread)				(truncate)
> > ext4_end_io_work()
> >   ext4_do_flush_completed_IO()
> >     spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> >     dump_completed_IO(inode);
> >     list_replace_init(&ei->i_completed_io_list, &unwritten);
> >     spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> > 
> > 					ext4_flush_completed_IO()
> > 					  ext4_do_flush_completed_IO()
> > 					    - sees empty i_completed_io_list
> > 					     => exits
> > 
> >   But we still have some conversions pending in 'unwritten' list. What am
> > I missing?
> Indeed, I've simply missed that case. The case which result silently
> broke integrity sync ;(
> Thank you for spotting this. I'll be back with updated version.
  Umm, actually, I was thinking about it and ext4_flush_completed_IO()
seems to be unnecessary in fsync these days. We don't call aio_complete()
until we perform the conversion so what fsync does to such IO is undefined.
Such optimization is a separate matter though.

But for truncate or punch hole, it is critical that all conversions are
really flushed.

								Honza