linux-ext4 - Re: [PATCH 4/4] ext4: serialize truncate with owerwrite DIO workers

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Date:	Wed, 05 Sep 2012 20:59:09 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org, jack@...e.cz
Subject: Re: [PATCH 4/4] ext4: serialize truncate with owerwrite DIO workers

On Wed, 5 Sep 2012 17:49:20 +0200, Jan Kara <jack@...e.cz> wrote:
> On Tue 04-09-12 21:36:54, Dmitry Monakhov wrote:
> > Jan Kara have spotted interesting issue:
> > There are  potential data corruption issue with  direct IO overwrites
> > racing with truncate:
> >  Like:
> >   dio write                      truncate_task
> >   ->ext4_ext_direct_IO
> >    ->overwrite == 1
> >     ->down_read(&EXT4_I(inode)->i_data_sem);
> >     ->mutex_unlock(&inode->i_mutex);
> >                                ->ext4_setattr()
> >                                 ->inode_dio_wait()
> >                                 ->truncate_setsize()
> >                                 ->ext4_truncate()
> >                                  ->down_write(&EXT4_I(inode)->i_data_sem);
> >     ->__blockdev_direct_IO
> >      ->ext4_get_block
> >      ->submit_io()
> >     ->up_read(&EXT4_I(inode)->i_data_sem);
> >                                  # truncate data blocks, allocate them to
> >                                  # other inode - bad stuff happens because
> >                                  # dio is still in flight.
> > 
> > In order to serialize with truncate dio worker should grab extra i_dio_count
> > reference before drop i_mutex.
>   Thanks for the patch. You can add:
> Reviewed-by: Jan Kara <jack@...e.cz>
I'm Sorry, but unfortunately in two line patch i've done one mistake :( 
because inode_dio_done() should be before i_mutex will be retaken
otherwise following deadlock happen

ext4_setattr                       ext4_direct_io
                                   mutex_unlock
                                   atomic_inc(inode->i_dio_count)
  mutex_lock(i_mutex)
  inode_dio_wait(inode)  ->BLOCK
                        DEADLOCK<- mutex_lock(i_mutex)
                                   inode_dio_done()

So i'll add your review sing to updated version if you don't mind.
> 								Honza
> > Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> > ---
> >  fs/ext4/inode.c |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 5a75908..9725acb 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3035,6 +3035,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
> >  		overwrite = *((int *)iocb->private);
> >  
> >  		if (overwrite) {
> > +			atomic_inc(&inode->i_dio_count);
> >  			down_read(&EXT4_I(inode)->i_data_sem);
> >  			mutex_unlock(&inode->i_mutex);
> >  		}
> > @@ -3134,6 +3135,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
> >  		if (overwrite) {
> >  			up_read(&EXT4_I(inode)->i_data_sem);
> >  			mutex_lock(&inode->i_mutex);
> > +			inode_dio_done(inode);
> >  		}
> >  
> >  		return ret;
> > -- 
> > 1.7.7.6
> > 
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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