writeback: do not remove bdi from bdi_list

writeback: do not remove bdi from bdi_list

The forker thread removes bdis from 'bdi_list' before forking the bdi thread.
But this is wrong for at least 2 reasons.

Reason #1: if we temporary remove a bdi from the list, we may miss works which
           would otherwise be given to us.

Reason #2: this is racy; indeed, 'bdi_wb_shutdown()' expects that bdis are
           always in the 'bdi_list' (see 'bdi_remove_from_list()'), and when
           it races with the forker thread, it can shut down the bdi thread
           at the same time as the forker creates it.

This patch makes sure the forker thread never removes bdis from 'bdi_list'
(which was suggested by Christoph Hellwig).

In order to make sure that we do not race with 'bdi_wb_shutdown()', we have to
hold the 'bdi_lock' while walking the 'bdi_list' and setting the 'BDI_pending'
flag.

NOTE! The error path is interesting. Currently, when we fail to create a bdi
thread, we move the bdi to the tail of 'bdi_list'. But if we never remove the
bdi from the list, we cannot move it to the tail either, because then we can
mess up the RCU readers which walk the list. And also, we'll have the race
described above in "Reason #2".

But I not think that adding to the tail is any important so I just do not do
that.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>