blkcg: Restructure blkg_conf_prep() and friends

We want to support lazy init of rq-qos policies so that iolatency is enabled
lazily on configuration instead of gendisk initialization. The way blkg
config helpers are structured now is a bit awkward for that. Let's
restructure:

* blkcg_conf_open_bdev() is renamed to blkg_conf_open_bdev(). The blkcg_
  prefix was used because the bdev opening step is blkg-independent.
  However, the distinction is too subtle and confuses more than helps. Let's
  switch to blkg prefix so that it's consistent with the type and other
  helper names.

* struct blkg_conf_ctx now remembers the original input string and is always
  initialized by the new blkg_conf_init().

* blkg_conf_open_bdev() is updated to take a pointer to blkg_conf_ctx like
  blkg_conf_prep() and can be called multiple times safely. Instead of
  modifying the double pointer to input string directly,
  blkg_conf_open_bdev() now sets blkg_conf_ctx->body.

* blkg_conf_finish() is renamed to blkg_conf_exit() for symmetry and now
  must be called on all blkg_conf_ctx's which were initialized with
  blkg_conf_init().

Combined, this allows the users to either open the bdev first or do it
altogether with blkg_conf_prep() which will help implementing lazy init of
rq-qos policies.

blkg_conf_init/exit() will also be used implement synchronization against
device removal. This is necessary because iolat / iocost are configured
through cgroupfs instead of one of the files under /sys/block/DEVICE. As
cgroupfs operations aren't synchronized with block layer, the lazy init and
other configuration operations may race against device removal. This patch
makes blkg_conf_init/exit() used consistently for all cgroup-orginating
configurations making them a good place to implement explicit
synchronization.

Users are updated accordingly. No behavior change is intended by this patch.

v2: bfq wasn't updated in v1 causing a build error. Fixed.

v3: Update the description to include future use of blkg_conf_init/exit() as
    synchronization points.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Yu Kuai <yukuai1@huaweicloud.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230413000649.115785-3-tj@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 10 insertions, 6 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 47e9d8be68f3..9bac95343ba0 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1368,9 +1368,11 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	int ret;
 	u64 v;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, &ctx);
 	if (ret)
-		return ret;
+		goto out_finish;
 
 	ret = -EINVAL;
 	if (sscanf(ctx.body, "%llu", &v) != 1)
@@ -1389,7 +1391,7 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of,
 	tg_conf_updated(tg, false);
 	ret = 0;
 out_finish:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 
@@ -1561,9 +1563,11 @@ static ssize_t tg_set_limit(struct kernfs_open_file *of,
 	int ret;
 	int index = of_cft(of)->private;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, &ctx);
 	if (ret)
-		return ret;
+		goto out_finish;
 
 	tg = blkg_to_tg(ctx.blkg);
 	tg_update_carryover(tg);
@@ -1662,7 +1666,7 @@ static ssize_t tg_set_limit(struct kernfs_open_file *of,
 		tg->td->limit_valid[LIMIT_LOW]);
 	ret = 0;
 out_finish:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }