bfq: Get rid of __bio_blkcg() usage

BFQ usage of __bio_blkcg() is a relict from the past. Furthermore if bio
would not be associated with any blkcg, the usage of __bio_blkcg() in
BFQ is prone to races with the task being migrated between cgroups as
__bio_blkcg() calls at different places could return different blkcgs.

Convert BFQ to the new situation where bio->bi_blkg is initialized in
bio_set_dev() and thus practically always valid. This allows us to save
blkcg_gq lookup and noticeably simplify the code.

CC: stable@vger.kernel.org
Fixes: 0fe061b9f03c ("blkcg: fix ref count issue with bio_blkcg() using task_css")
Tested-by: "yukuai (C)" <yukuai3@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220401102752.8599-8-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 879380c..32d2c2a 100644 (file)
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -586,27 +586,11 @@ static void bfq_group_set_parent(struct bfq_group *bfqg,
        entity->sched_data = &parent->sched_data;
 }
 
-static struct bfq_group *bfq_lookup_bfqg(struct bfq_data *bfqd,
-                                        struct blkcg *blkcg)
+static void bfq_link_bfqg(struct bfq_data *bfqd, struct bfq_group *bfqg)
 {
-       struct blkcg_gq *blkg;
-
-       blkg = blkg_lookup(blkcg, bfqd->queue);
-       if (likely(blkg))
-               return blkg_to_bfqg(blkg);
-       return NULL;
-}
-
-struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
-                                    struct blkcg *blkcg)
-{
-       struct bfq_group *bfqg, *parent;
+       struct bfq_group *parent;
        struct bfq_entity *entity;
 
-       bfqg = bfq_lookup_bfqg(bfqd, blkcg);
-       if (unlikely(!bfqg))
-               return NULL;
-
        /*
         * Update chain of bfq_groups as we might be handling a leaf group
         * which, along with some of its relatives, has not been hooked yet
@@ -623,8 +607,15 @@ struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
                        bfq_group_set_parent(curr_bfqg, parent);
                }
        }
+}
 
-       return bfqg;
+struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio)
+{
+       struct blkcg_gq *blkg = bio->bi_blkg;
+
+       if (!blkg)
+               return bfqd->root_group;
+       return blkg_to_bfqg(blkg);
 }
 
 /**
@@ -714,25 +705,15 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
  * Move bic to blkcg, assuming that bfqd->lock is held; which makes
  * sure that the reference to cgroup is valid across the call (see
  * comments in bfq_bic_update_cgroup on this issue)
- *
- * NOTE: an alternative approach might have been to store the current
- * cgroup in bfqq and getting a reference to it, reducing the lookup
- * time here, at the price of slightly more complex code.
  */
-static struct bfq_group *__bfq_bic_change_cgroup(struct bfq_data *bfqd,
-                                               struct bfq_io_cq *bic,
-                                               struct blkcg *blkcg)
+static void *__bfq_bic_change_cgroup(struct bfq_data *bfqd,
+                                    struct bfq_io_cq *bic,
+                                    struct bfq_group *bfqg)
 {
        struct bfq_queue *async_bfqq = bic_to_bfqq(bic, 0);
        struct bfq_queue *sync_bfqq = bic_to_bfqq(bic, 1);
-       struct bfq_group *bfqg;
        struct bfq_entity *entity;
 
-       bfqg = bfq_find_set_group(bfqd, blkcg);
-
-       if (unlikely(!bfqg))
-               bfqg = bfqd->root_group;
-
        if (async_bfqq) {
                entity = &async_bfqq->entity;
 
@@ -784,20 +765,24 @@ static struct bfq_group *__bfq_bic_change_cgroup(struct bfq_data *bfqd,
 void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio)
 {
        struct bfq_data *bfqd = bic_to_bfqd(bic);
-       struct bfq_group *bfqg = NULL;
+       struct bfq_group *bfqg = bfq_bio_bfqg(bfqd, bio);
        uint64_t serial_nr;
 
-       rcu_read_lock();
-       serial_nr = __bio_blkcg(bio)->css.serial_nr;
+       serial_nr = bfqg_to_blkg(bfqg)->blkcg->css.serial_nr;
 
        /*
         * Check whether blkcg has changed.  The condition may trigger
         * spuriously on a newly created cic but there's no harm.
         */
        if (unlikely(!bfqd) || likely(bic->blkcg_serial_nr == serial_nr))
-               goto out;
+               return;
 
-       bfqg = __bfq_bic_change_cgroup(bfqd, bic, __bio_blkcg(bio));
+       /*
+        * New cgroup for this process. Make sure it is linked to bfq internal
+        * cgroup hierarchy.
+        */
+       bfq_link_bfqg(bfqd, bfqg);
+       __bfq_bic_change_cgroup(bfqd, bic, bfqg);
        /*
         * Update blkg_path for bfq_log_* functions. We cache this
         * path, and update it here, for the following
@@ -850,8 +835,6 @@ void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio)
         */
        blkg_path(bfqg_to_blkg(bfqg), bfqg->blkg_path, sizeof(bfqg->blkg_path));
        bic->blkcg_serial_nr = serial_nr;
-out:
-       rcu_read_unlock();
 }
 
 /**
@@ -1469,7 +1452,7 @@ void bfq_end_wr_async(struct bfq_data *bfqd)
        bfq_end_wr_async_queues(bfqd, bfqd->root_group);
 }
 
-struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd, struct blkcg *blkcg)
+struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio)
 {
        return bfqd->root_group;
 }

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index d7cf930..e47c75f 100644 (file)
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5726,14 +5726,7 @@ static struct bfq_queue *bfq_get_queue(struct bfq_data *bfqd,
        struct bfq_queue *bfqq;
        struct bfq_group *bfqg;
 
-       rcu_read_lock();
-
-       bfqg = bfq_find_set_group(bfqd, __bio_blkcg(bio));
-       if (!bfqg) {
-               bfqq = &bfqd->oom_bfqq;
-               goto out;
-       }
-
+       bfqg = bfq_bio_bfqg(bfqd, bio);
        if (!is_sync) {
                async_bfqq = bfq_async_queue_prio(bfqd, bfqg, ioprio_class,
                                                  ioprio);
@@ -5779,8 +5772,6 @@ out:
 
        if (bfqq != &bfqd->oom_bfqq && is_sync && !respawn)
                bfqq = bfq_do_or_sched_stable_merge(bfqd, bfqq, bic);
-
-       rcu_read_unlock();
        return bfqq;
 }
 

diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 4664e2f..978ef5d 100644 (file)
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -1009,8 +1009,7 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 void bfq_init_entity(struct bfq_entity *entity, struct bfq_group *bfqg);
 void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio);
 void bfq_end_wr_async(struct bfq_data *bfqd);
-struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
-                                    struct blkcg *blkcg);
+struct bfq_group *bfq_bio_bfqg(struct bfq_data *bfqd, struct bio *bio);
 struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg);
 struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
 struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node);