Re: [PATCH 4/4] writeback: Avoid iput() from flusher thread

[Jan Kara <jack@suse.cz>]

On Mon 19-03-12 04:55:15, Christoph Hellwig wrote:
> On Fri, Mar 09, 2012 at 10:02:28AM +0100, Jan Kara wrote:
> > Doing iput() from flusher thread (writeback_sb_inodes()) can create problems
> > because iput() can do a lot of work - for example truncate the inode if it's
> > the last iput on unlinked file. Some filesystems (e.g. ubifs) may need to
> > allocate blocks during truncate (due to their COW nature) and in some cases
> > they thus need to flush dirty data from truncate to reduce uncertainty in the
> > amount of free space. This effectively creates a deadlock.
> > 
> > We get rid of iput() in flusher thread by using the fact that I_SYNC inode
> > flag effectively pins the inode in memory. So if we take care to either hold
> > i_lock or have I_SYNC set, we can get away without taking inode reference
> > in writeback_sb_inodes().
> > 
> > As a side effect, we also fix possible use-after-free in wb_writeback() because
> > inode_wait_for_writeback() call could try to reacquire i_lock on the inode that
> > was already free.
> > 
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/fs-writeback.c         |   38 ++++++++++++++++++++++++--------------
> >  fs/inode.c                |   11 ++++++++++-
> >  include/linux/fs.h        |    7 ++++---
> >  include/linux/writeback.h |    7 +------
> >  4 files changed, 39 insertions(+), 24 deletions(-)
> > 
> > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> > index 1e8bf44..f9f9b61 100644
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@ -325,19 +325,21 @@ static int write_inode(struct inode *inode, struct writeback_control *wbc)
> >  }
> >  
> >  /*
> > - * Wait for writeback on an inode to complete.
> > + * Wait for writeback on an inode to complete. Called with i_lock held.
> > + * Return 1 if we dropped i_lock and waited, 0 is returned otherwise.
> >   */
> > -static void inode_wait_for_writeback(struct inode *inode)
> > +int __must_check inode_wait_for_writeback(struct inode *inode)
> >  {
> >  	DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
> >  	wait_queue_head_t *wqh;
> >  
> >  	wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
> > +	if (inode->i_state & I_SYNC) {
> >  		spin_unlock(&inode->i_lock);
> >  		__wait_on_bit(wqh, &wq, inode_wait, TASK_UNINTERRUPTIBLE);
> > +		return 1;
> >  	}
> > +	return 0;
> 
> This is a horribly ugl primitive.
> 
> I'd rather add a
> 
> void inode_wait_for_writeback(struct inode *inode)
> {
>  	DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
>  	wait_queue_head_t *wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
> 
> 	__wait_on_bit(wqh, &wq, inode_wait, TASK_UNINTERRUPTIBLE);
> }
> 
> and opencode all the locking ad I_SYNC checking logic in the callers.
  I agree the primitive is ugly. And actually it is buggy the way I wrote
it. It should have been:
  __wait_on_bit(wqh, &wq, isync_wait, TASK_UNINTERRUPTIBLE);

where isync_wait is:

int isync_wait(void *word)
{
	struct inode *inode = container_of(word, struct inode, i_state);

	spin_unlock(&inode->i_lock);
	schedule();
	return 1;
}

  The problem is i_lock pins the inode for us in some cases. So once we
drop i_lock, inode can go away so we cannot test the bit anymore.

But there are just two places where we really need this. So maybe I can
just opencode it there and for others use normal obvious variant.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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