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>

3 files changed, 10 insertions, 2 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;
 
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index c3fedd265d18..3b724fa2b68d 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1059,6 +1059,8 @@ static void cached_dev_free(struct closure *cl)
         cancel_delayed_work_sync(&dc->writeback_rate_update);
         if (!IS_ERR_OR_NULL(dc->writeback_thread))
                 kthread_stop(dc->writeback_thread);
+        if (dc->writeback_write_wq)
+                destroy_workqueue(dc->writeback_write_wq);
 
         mutex_lock(&bch_register_lock);
 
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 5abe6472b66b..34131439e28c 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -187,7 +187,7 @@ static void write_dirty(struct closure *cl)
 
         closure_bio_submit(&io->bio, cl);
 
-        continue_at(cl, write_dirty_finish, system_wq);
+        continue_at(cl, write_dirty_finish, io->dc->writeback_write_wq);
 }
 
 static void read_dirty_endio(struct bio *bio)
@@ -207,7 +207,7 @@ static void read_dirty_submit(struct closure *cl)
 
         closure_bio_submit(&io->bio, cl);
 
-        continue_at(cl, write_dirty, system_wq);
+        continue_at(cl, write_dirty, io->dc->writeback_write_wq);
 }
 
 static void read_dirty(struct cached_dev *dc)
@@ -516,6 +516,11 @@ void bch_cached_dev_writeback_init(struct cached_dev *dc)
 
 int bch_cached_dev_writeback_start(struct cached_dev *dc)
 {
+        dc->writeback_write_wq = alloc_workqueue("bcache_writeback_wq",
+                                                WQ_MEM_RECLAIM, 0);
+        if (!dc->writeback_write_wq)
+                return -ENOMEM;
+
         dc->writeback_thread = kthread_create(bch_writeback_thread, dc,
                                               "bcache_writeback");
         if (IS_ERR(dc->writeback_thread))