Re: [PATCH 17/19] bcache: fix for gc and write-back race

Coly Li
2017-08-03T16:20:18Z

On 2017/7/1 上午4:43, bcache@lists.ewheeler.net wrote:
> From: Tang Junhui <tang.junhui@zte.com.cn>
>
> 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.
>
> Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
> Cc: stable@vger.kernel.org

Add a per-cached device work queue is a good idea, it's OK to me.

Acked-by: Coly Li <colyli@suse.de>

Thansk.

Coly

> ---
> drivers/md/bcache/bcache.h | 1 +
> drivers/md/bcache/super.c | 2 ++
> drivers/md/bcache/writeback.c | 8 ++++++--
> 3 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 44123e4..deb0a6c 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 e06641e..24cb9b7 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1063,6 +1063,8 @@ 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 4104eaa..4bc5daa 100644
> --- a/drivers/md/bcache/writeback.c
> +++ b/drivers/md/bcache/writeback.c
> @@ -189,7 +189,7 @@ 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)
> @@ -209,7 +209,7 @@ 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)
> @@ -527,6 +527,10 @@ 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))
>