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

[Christoph Hellwig]

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@xxxxxxx>
> ---
>  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.

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