Commit 6147305c authored May 03, 2018 by Coly Li Committed by Jens Axboe May 03, 2018

bcache: set CACHE_SET_IO_DISABLE in bch_cached_dev_error()

Commit c7b7bd07 ("bcache: add io_disable to struct cached_dev") tries
to stop bcache device by calling bcache_device_stop() when too many I/O
errors happened on backing device. But if there is internal I/O happening
on cache device (writeback scan, garbage collection, etc), a regular I/O
request triggers the internal I/Os may still holds a refcount of dc->count,
and the refcount may only be dropped after the internal I/O stopped.

By this patch, bch_cached_dev_error() will check if the backing device is
attached to a cache set, if yes that CACHE_SET_IO_DISABLE will be set to
flags of this cache set. Then internal I/Os on cache device will be
rejected and stopped immediately, and the bcache device can be stopped.

For people who are not familiar with the interesting refcount dependance,
let me explain a bit more how the fix works. Example the writeback thread
will scan cache device for dirty data writeback purpose. Before it stopps,
it holds a refcount of dc->count. When CACHE_SET_IO_DISABLE bit is set,
the internal I/O will stopped and the while-loop in bch_writeback_thread()
quits and calls cached_dev_put() to drop dc->count. If this is the last
refcount to drop, then cached_dev_detach_finish() will be called. In this
call back function, in turn closure_put(dc->disk.cl) is called to drop a
refcount of closure dc->disk.cl. If this is the last refcount of this
closure to drop, then cached_dev_flush() will be called. Then the cached
device is freed. So if CACHE_SET_IO_DISABLE is not set, the bache device
can not be stopped until all inernal cache device I/O stopped. For large
size cache device, and writeback thread competes locks with gc thread,
there might be a quite long time to wait.

Fixes: c7b7bd07 ("bcache: add io_disable to struct cached_dev")
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 6e916a7e

drivers/md/bcache/super.c

+17 −0

Original line number	Diff line number	Diff line
		@@ -1369,6 +1369,8 @@ int bch_flash_dev_create(struct cache_set *c, uint64_t size)

		bool bch_cached_dev_error(struct cached_dev *dc)
		{
		struct cache_set *c;

		if (!dc \|\| test_bit(BCACHE_DEV_CLOSING, &dc->disk.flags))
		return false;

		@@ -1379,6 +1381,21 @@ bool bch_cached_dev_error(struct cached_dev *dc)
		pr_err("stop %s: too many IO errors on backing device %s\n",
		dc->disk.disk->disk_name, dc->backing_dev_name);

		/*
		* If the cached device is still attached to a cache set,
		* even dc->io_disable is true and no more I/O requests
		* accepted, cache device internal I/O (writeback scan or
		* garbage collection) may still prevent bcache device from
		* being stopped. So here CACHE_SET_IO_DISABLE should be
		* set to c->flags too, to make the internal I/O to cache
		* device rejected and stopped immediately.
		* If c is NULL, that means the bcache device is not attached
		* to any cache set, then no CACHE_SET_IO_DISABLE bit to set.
		*/
		c = dc->disk.c;
		if (c && test_and_set_bit(CACHE_SET_IO_DISABLE, &c->flags))
		pr_info("CACHE_SET_IO_DISABLE already set");

		bcache_device_stop(&dc->disk);
		return true;
		}