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>

block/bfq-cgroup.c

@@ -1105,9 +1105,11 @@ static ssize_t bfq_io_set_device_weight(struct kernfs_open_file *of,
 	struct bfq_group *bfqg;
 	u64 v;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_bfq, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_bfq, &ctx);
 	if (ret)
-		return ret;
+		goto out;
 
 	if (sscanf(ctx.body, "%llu", &v) == 1) {
 		/* require "default" on dfl */
@@ -1129,7 +1131,7 @@ static ssize_t bfq_io_set_device_weight(struct kernfs_open_file *of,
 		ret = 0;
 	}
 out:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 

block/blk-cgroup.c

@@ -653,69 +653,93 @@ u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v)
 EXPORT_SYMBOL_GPL(__blkg_prfill_u64);
 
 /**
- * blkcg_conf_open_bdev - parse and open bdev for per-blkg config update
- * @inputp: input string pointer
+ * blkg_conf_init - initialize a blkg_conf_ctx
+ * @ctx: blkg_conf_ctx to initialize
+ * @input: input string
  *
- * Parse the device node prefix part, MAJ:MIN, of per-blkg config update
- * from @input and get and return the matching bdev.  *@inputp is
- * updated to point past the device node prefix.  Returns an ERR_PTR()
- * value on error.
- *
- * Use this function iff blkg_conf_prep() can't be used for some reason.
+ * Initialize @ctx which can be used to parse blkg config input string @input.
+ * Once initialized, @ctx can be used with blkg_conf_open_bdev() and
+ * blkg_conf_prep(), and must be cleaned up with blkg_conf_exit().
  */
-struct block_device *blkcg_conf_open_bdev(char **inputp)
+void blkg_conf_init(struct blkg_conf_ctx *ctx, char *input)
 {
-	char *input = *inputp;
+	*ctx = (struct blkg_conf_ctx){ .input = input };
+}
+EXPORT_SYMBOL_GPL(blkg_conf_init);
+
+/**
+ * blkg_conf_open_bdev - parse and open bdev for per-blkg config update
+ * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
+ *
+ * Parse the device node prefix part, MAJ:MIN, of per-blkg config update from
+ * @ctx->input and get and store the matching bdev in @ctx->bdev. @ctx->body is
+ * set to point past the device node prefix.
+ *
+ * This function may be called multiple times on @ctx and the extra calls become
+ * NOOPs. blkg_conf_prep() implicitly calls this function. Use this function
+ * explicitly if bdev access is needed without resolving the blkcg / policy part
+ * of @ctx->input. Returns -errno on error.
+ */
+int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
+{
+	char *input = ctx->input;
 	unsigned int major, minor;
 	struct block_device *bdev;
 	int key_len;
 
+	if (ctx->bdev)
+		return 0;
+
 	if (sscanf(input, "%u:%u%n", &major, &minor, &key_len) != 2)
-		return ERR_PTR(-EINVAL);
+		return -EINVAL;
 
 	input += key_len;
 	if (!isspace(*input))
-		return ERR_PTR(-EINVAL);
+		return -EINVAL;
 	input = skip_spaces(input);
 
 	bdev = blkdev_get_no_open(MKDEV(major, minor));
 	if (!bdev)
-		return ERR_PTR(-ENODEV);
+		return -ENODEV;
 	if (bdev_is_partition(bdev)) {
 		blkdev_put_no_open(bdev);
-		return ERR_PTR(-ENODEV);
+		return -ENODEV;
 	}
 
-	*inputp = input;
-	return bdev;
+	ctx->body = input;
+	ctx->bdev = bdev;
+	return 0;
 }
 
 /**
  * blkg_conf_prep - parse and prepare for per-blkg config update
  * @blkcg: target block cgroup
  * @pol: target policy
- * @input: input string
- * @ctx: blkg_conf_ctx to be filled
+ * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
- * Parse per-blkg config update from @input and initialize @ctx with the
- * result.  @ctx->blkg points to the blkg to be updated and @ctx->body the
- * part of @input following MAJ:MIN.  This function returns with queue lock
- * held and must be paired with blkg_conf_finish().
+ * Parse per-blkg config update from @ctx->input and initialize @ctx
+ * accordingly. On success, @ctx->body points to the part of @ctx->input
+ * following MAJ:MIN, @ctx->bdev points to the target block device and
+ * @ctx->blkg to the blkg being configured.
+ *
+ * blkg_conf_open_bdev() may be called on @ctx beforehand. On success, this
+ * function returns with queue lock held and must be followed by
+ * blkg_conf_exit().
  */
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
-		   char *input, struct blkg_conf_ctx *ctx)
+		   struct blkg_conf_ctx *ctx)
 	__acquires(&bdev->bd_queue->queue_lock)
 {
-	struct block_device *bdev;
 	struct gendisk *disk;
 	struct request_queue *q;
 	struct blkcg_gq *blkg;
 	int ret;
 
-	bdev = blkcg_conf_open_bdev(&input);
-	if (IS_ERR(bdev))
-		return PTR_ERR(bdev);
-	disk = bdev->bd_disk;
+	ret = blkg_conf_open_bdev(ctx);
+	if (ret)
+		return ret;
+
+	disk = ctx->bdev->bd_disk;
 	q = disk->queue;
 
 	/*
@@ -793,9 +817,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	}
 success:
 	blk_queue_exit(q);
-	ctx->bdev = bdev;
 	ctx->blkg = blkg;
-	ctx->body = input;
 	return 0;
 
 fail_preloaded:
@@ -805,7 +827,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 fail_exit_queue:
 	blk_queue_exit(q);
 fail:
-	blkdev_put_no_open(bdev);
 	/*
 	 * If queue was bypassing, we should retry.  Do so after a
 	 * short msleep().  It isn't strictly necessary but queue
@@ -821,19 +842,27 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 EXPORT_SYMBOL_GPL(blkg_conf_prep);
 
 /**
- * blkg_conf_finish - finish up per-blkg config update
- * @ctx: blkg_conf_ctx initialized by blkg_conf_prep()
+ * blkg_conf_exit - clean up per-blkg config update
+ * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
  *
- * Finish up after per-blkg config update.  This function must be paired
- * with blkg_conf_prep().
+ * Clean up after per-blkg config update. This function must be called on all
+ * blkg_conf_ctx's initialized with blkg_conf_init().
  */
-void blkg_conf_finish(struct blkg_conf_ctx *ctx)
+void blkg_conf_exit(struct blkg_conf_ctx *ctx)
 	__releases(&ctx->bdev->bd_queue->queue_lock)
 {
-	spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
-	blkdev_put_no_open(ctx->bdev);
+	if (ctx->blkg) {
+		spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
+		ctx->blkg = NULL;
+	}
+
+	if (ctx->bdev) {
+		blkdev_put_no_open(ctx->bdev);
+		ctx->body = NULL;
+		ctx->bdev = NULL;
+	}
 }
-EXPORT_SYMBOL_GPL(blkg_conf_finish);
+EXPORT_SYMBOL_GPL(blkg_conf_exit);
 
 static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
 {

block/blk-cgroup.h

@@ -206,15 +206,17 @@ void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
 u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v);
 
 struct blkg_conf_ctx {
+	char				*input;
+	char				*body;
 	struct block_device		*bdev;
 	struct blkcg_gq			*blkg;
-	char				*body;
 };
 
-struct block_device *blkcg_conf_open_bdev(char **inputp);
+void blkg_conf_init(struct blkg_conf_ctx *ctx, char *input);
+int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx);
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
-		   char *input, struct blkg_conf_ctx *ctx);
-void blkg_conf_finish(struct blkg_conf_ctx *ctx);
+		   struct blkg_conf_ctx *ctx);
+void blkg_conf_exit(struct blkg_conf_ctx *ctx);
 
 /**
  * bio_issue_as_root_blkg - see if this bio needs to be issued as root blkg

block/blk-iocost.c

@@ -3106,9 +3106,11 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
 		return nbytes;
 	}
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, &ctx);
 	if (ret)
-		return ret;
+		goto err;
 
 	iocg = blkg_to_iocg(ctx.blkg);
 
@@ -3127,12 +3129,14 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
 	weight_updated(iocg, &now);
 	spin_unlock(&iocg->ioc->lock);
 
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return nbytes;
 
 einval:
-	blkg_conf_finish(&ctx);
-	return -EINVAL;
+	ret = -EINVAL;
+err:
+	blkg_conf_exit(&ctx);
+	return ret;
 }
 
 static u64 ioc_qos_prfill(struct seq_file *sf, struct blkg_policy_data *pd,
@@ -3189,19 +3193,22 @@ static const match_table_t qos_tokens = {
 static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 			     size_t nbytes, loff_t off)
 {
-	struct block_device *bdev;
+	struct blkg_conf_ctx ctx;
 	struct gendisk *disk;
 	struct ioc *ioc;
 	u32 qos[NR_QOS_PARAMS];
 	bool enable, user;
-	char *p;
+	char *body, *p;
 	int ret;
 
-	bdev = blkcg_conf_open_bdev(&input);
-	if (IS_ERR(bdev))
-		return PTR_ERR(bdev);
+	blkg_conf_init(&ctx, input);
 
-	disk = bdev->bd_disk;
+	ret = blkg_conf_open_bdev(&ctx);
+	if (ret)
+		goto err;
+
+	body = ctx.body;
+	disk = ctx.bdev->bd_disk;
 	if (!queue_is_mq(disk->queue)) {
 		ret = -EOPNOTSUPP;
 		goto err;
@@ -3223,7 +3230,7 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 	enable = ioc->enabled;
 	user = ioc->user_qos_params;
 
-	while ((p = strsep(&input, " \t\n"))) {
+	while ((p = strsep(&body, " \t\n"))) {
 		substring_t args[MAX_OPT_ARGS];
 		char buf[32];
 		int tok;
@@ -3313,7 +3320,7 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 	blk_mq_unquiesce_queue(disk->queue);
 	blk_mq_unfreeze_queue(disk->queue);
 
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return nbytes;
 einval:
 	spin_unlock_irq(&ioc->lock);
@@ -3323,7 +3330,7 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
 
 	ret = -EINVAL;
 err:
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return ret;
 }
 
@@ -3376,19 +3383,22 @@ static const match_table_t i_lcoef_tokens = {
 static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 				    size_t nbytes, loff_t off)
 {
-	struct block_device *bdev;
+	struct blkg_conf_ctx ctx;
 	struct request_queue *q;
 	struct ioc *ioc;
 	u64 u[NR_I_LCOEFS];
 	bool user;
-	char *p;
+	char *body, *p;
 	int ret;
 
-	bdev = blkcg_conf_open_bdev(&input);
-	if (IS_ERR(bdev))
-		return PTR_ERR(bdev);
+	blkg_conf_init(&ctx, input);
 
-	q = bdev_get_queue(bdev);
+	ret = blkg_conf_open_bdev(&ctx);
+	if (ret)
+		goto err;
+
+	body = ctx.body;
+	q = bdev_get_queue(ctx.bdev);
 	if (!queue_is_mq(q)) {
 		ret = -EOPNOTSUPP;
 		goto err;
@@ -3396,7 +3406,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 
 	ioc = q_to_ioc(q);
 	if (!ioc) {
-		ret = blk_iocost_init(bdev->bd_disk);
+		ret = blk_iocost_init(ctx.bdev->bd_disk);
 		if (ret)
 			goto err;
 		ioc = q_to_ioc(q);
@@ -3409,7 +3419,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 	memcpy(u, ioc->params.i_lcoefs, sizeof(u));
 	user = ioc->user_cost_model;
 
-	while ((p = strsep(&input, " \t\n"))) {
+	while ((p = strsep(&body, " \t\n"))) {
 		substring_t args[MAX_OPT_ARGS];
 		char buf[32];
 		int tok;
@@ -3456,7 +3466,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 	blk_mq_unquiesce_queue(q);
 	blk_mq_unfreeze_queue(q);
 
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return nbytes;
 
 einval:
@@ -3467,7 +3477,7 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
 
 	ret = -EINVAL;
 err:
-	blkdev_put_no_open(bdev);
+	blkg_conf_exit(&ctx);
 	return ret;
 }
 

block/blk-iolatency.c

@@ -836,9 +836,11 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 	u64 oldval;
 	int ret;
 
-	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, buf, &ctx);
+	blkg_conf_init(&ctx, buf);
+
+	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, &ctx);
 	if (ret)
-		return ret;
+		goto out;
 
 	iolat = blkg_to_lat(ctx.blkg);
 	p = ctx.body;
@@ -874,7 +876,7 @@ static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
 		iolatency_clear_scaling(blkg);
 	ret = 0;
 out:
-	blkg_conf_finish(&ctx);
+	blkg_conf_exit(&ctx);
 	return ret ?: nbytes;
 }
 

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;
 }