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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