bcache: fix for gc and write-back race

gc and write-back get raced (see the email "bcache get stucked" I sended
before):
gc thread                               write-back thread
|                                       |bch_writeback_thread()
|bch_gc_thread()                        |
|                                       |==>read_dirty()
|==>bch_btree_gc()                      |
|==>btree_root() //get btree root       |
|                //node write locker    |
|==>bch_btree_gc_root()                 |
|                                       |==>read_dirty_submit()
|                                       |==>write_dirty()
|                                       |==>continue_at(cl,
|                                       |               write_dirty_finish,
|                                       |               system_wq);
|                                       |==>write_dirty_finish()//excute
|                                       |               //in system_wq
|                                       |==>bch_btree_insert()
|                                       |==>bch_btree_map_leaf_nodes()
|                                       |==>__bch_btree_map_nodes()
|                                       |==>btree_root //try to get btree
|                                       |              //root node read
|                                       |              //lock
|                                       |-----stuck here
|==>bch_btree_set_root()
|==>bch_journal_meta()
|==>bch_journal()
|==>journal_try_write()
|==>journal_write_unlocked() //journal_full(&c->journal)
|                            //condition satisfied
|==>continue_at(cl, journal_write, system_wq); //try to excute
|                               //journal_write in system_wq
|                               //but work queue is excuting
|                               //write_dirty_finish()
|==>closure_sync(); //wait journal_write execute
|                   //over and wake up gc,
|-------------stuck here
|==>release root node write locker

This patch alloc a separate work-queue for write-back thread to avoid such
race.

(Commit log re-organized by Coly Li to pass checkpatch.pl checking)

Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
Acked-by: Coly Li <colyli@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index dee542fff68e..2ed9bd231d84 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -333,6 +333,7 @@ struct cached_dev {
 	/* Limit number of writeback bios in flight */
 	struct semaphore	in_flight;
 	struct task_struct	*writeback_thread;
+	struct workqueue_struct	*writeback_write_wq;
 
 	struct keybuf		writeback_keys;