diff options
Diffstat (limited to 'block/bfq-cgroup.c')
| -rw-r--r-- | block/bfq-cgroup.c | 116 |
1 files changed, 93 insertions, 23 deletions
diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c index c8a32fb345cf..78b2e0db4fb2 100644 --- a/block/bfq-cgroup.c +++ b/block/bfq-cgroup.c @@ -52,7 +52,7 @@ BFQG_FLAG_FNS(idling) BFQG_FLAG_FNS(empty) #undef BFQG_FLAG_FNS -/* This should be called with the queue_lock held. */ +/* This should be called with the scheduler lock held. */ static void bfqg_stats_update_group_wait_time(struct bfqg_stats *stats) { unsigned long long now; @@ -67,7 +67,7 @@ static void bfqg_stats_update_group_wait_time(struct bfqg_stats *stats) bfqg_stats_clear_waiting(stats); } -/* This should be called with the queue_lock held. */ +/* This should be called with the scheduler lock held. */ static void bfqg_stats_set_start_group_wait_time(struct bfq_group *bfqg, struct bfq_group *curr_bfqg) { @@ -81,7 +81,7 @@ static void bfqg_stats_set_start_group_wait_time(struct bfq_group *bfqg, bfqg_stats_mark_waiting(stats); } -/* This should be called with the queue_lock held. */ +/* This should be called with the scheduler lock held. */ static void bfqg_stats_end_empty_time(struct bfqg_stats *stats) { unsigned long long now; @@ -203,12 +203,30 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq) static void bfqg_get(struct bfq_group *bfqg) { - return blkg_get(bfqg_to_blkg(bfqg)); + bfqg->ref++; } void bfqg_put(struct bfq_group *bfqg) { - return blkg_put(bfqg_to_blkg(bfqg)); + bfqg->ref--; + + if (bfqg->ref == 0) + kfree(bfqg); +} + +static void bfqg_and_blkg_get(struct bfq_group *bfqg) +{ + /* see comments in bfq_bic_update_cgroup for why refcounting bfqg */ + bfqg_get(bfqg); + + blkg_get(bfqg_to_blkg(bfqg)); +} + +void bfqg_and_blkg_put(struct bfq_group *bfqg) +{ + bfqg_put(bfqg); + + blkg_put(bfqg_to_blkg(bfqg)); } void bfqg_stats_update_io_add(struct bfq_group *bfqg, struct bfq_queue *bfqq, @@ -312,7 +330,11 @@ void bfq_init_entity(struct bfq_entity *entity, struct bfq_group *bfqg) if (bfqq) { bfqq->ioprio = bfqq->new_ioprio; bfqq->ioprio_class = bfqq->new_ioprio_class; - bfqg_get(bfqg); + /* + * Make sure that bfqg and its associated blkg do not + * disappear before entity. + */ + bfqg_and_blkg_get(bfqg); } entity->parent = bfqg->my_entity; /* NULL for root group */ entity->sched_data = &bfqg->sched_data; @@ -399,6 +421,8 @@ struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, int node) return NULL; } + /* see comments in bfq_bic_update_cgroup for why refcounting */ + bfqg_get(bfqg); return &bfqg->pd; } @@ -426,7 +450,7 @@ void bfq_pd_free(struct blkg_policy_data *pd) struct bfq_group *bfqg = pd_to_bfqg(pd); bfqg_stats_exit(&bfqg->stats); - return kfree(bfqg); + bfqg_put(bfqg); } void bfq_pd_reset_stats(struct blkg_policy_data *pd) @@ -496,9 +520,10 @@ struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd, * Move @bfqq to @bfqg, deactivating it from its old group and reactivating * it on the new one. Avoid putting the entity on the old group idle tree. * - * Must be called under the queue lock; the cgroup owning @bfqg must - * not disappear (by now this just means that we are called under - * rcu_read_lock()). + * Must be called under the scheduler lock, to make sure that the blkg + * owning @bfqg does not disappear (see comments in + * bfq_bic_update_cgroup on guaranteeing the consistency of blkg + * objects). */ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq, struct bfq_group *bfqg) @@ -519,16 +544,12 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq, bfq_deactivate_bfqq(bfqd, bfqq, false, false); else if (entity->on_st) bfq_put_idle_entity(bfq_entity_service_tree(entity), entity); - bfqg_put(bfqq_group(bfqq)); + bfqg_and_blkg_put(bfqq_group(bfqq)); - /* - * Here we use a reference to bfqg. We don't need a refcounter - * as the cgroup reference will not be dropped, so that its - * destroy() callback will not be invoked. - */ entity->parent = bfqg->my_entity; entity->sched_data = &bfqg->sched_data; - bfqg_get(bfqg); + /* pin down bfqg and its associated blkg */ + bfqg_and_blkg_get(bfqg); if (bfq_bfqq_busy(bfqq)) { bfq_pos_tree_add_move(bfqd, bfqq); @@ -545,8 +566,9 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq, * @bic: the bic to move. * @blkcg: the blk-cgroup to move to. * - * Move bic to blkcg, assuming that bfqd->queue is locked; the caller - * has to make sure that the reference to cgroup is valid across the call. + * 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 @@ -604,6 +626,57 @@ void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio) goto out; bfqg = __bfq_bic_change_cgroup(bfqd, bic, bio_blkcg(bio)); + /* + * Update blkg_path for bfq_log_* functions. We cache this + * path, and update it here, for the following + * reasons. Operations on blkg objects in blk-cgroup are + * protected with the request_queue lock, and not with the + * lock that protects the instances of this scheduler + * (bfqd->lock). This exposes BFQ to the following sort of + * race. + * + * The blkg_lookup performed in bfq_get_queue, protected + * through rcu, may happen to return the address of a copy of + * the original blkg. If this is the case, then the + * bfqg_and_blkg_get performed in bfq_get_queue, to pin down + * the blkg, is useless: it does not prevent blk-cgroup code + * from destroying both the original blkg and all objects + * directly or indirectly referred by the copy of the + * blkg. + * + * On the bright side, destroy operations on a blkg invoke, as + * a first step, hooks of the scheduler associated with the + * blkg. And these hooks are executed with bfqd->lock held for + * BFQ. As a consequence, for any blkg associated with the + * request queue this instance of the scheduler is attached + * to, we are guaranteed that such a blkg is not destroyed, and + * that all the pointers it contains are consistent, while we + * are holding bfqd->lock. A blkg_lookup performed with + * bfqd->lock held then returns a fully consistent blkg, which + * remains consistent until this lock is held. + * + * Thanks to the last fact, and to the fact that: (1) bfqg has + * been obtained through a blkg_lookup in the above + * assignment, and (2) bfqd->lock is being held, here we can + * safely use the policy data for the involved blkg (i.e., the + * field bfqg->pd) to get to the blkg associated with bfqg, + * and then we can safely use any field of blkg. After we + * release bfqd->lock, even just getting blkg through this + * bfqg may cause dangling references to be traversed, as + * bfqg->pd may not exist any more. + * + * In view of the above facts, here we cache, in the bfqg, any + * blkg data we may need for this bic, and for its associated + * bfq_queue. As of now, we need to cache only the path of the + * blkg, which is used in the bfq_log_* functions. + * + * Finally, note that bfqg itself needs to be protected from + * destruction on the blkg_free of the original blkg (which + * invokes bfq_pd_free). We use an additional private + * refcounter for bfqg, to let it disappear only after no + * bfq_queue refers to it any longer. + */ + blkg_path(bfqg_to_blkg(bfqg), bfqg->blkg_path, sizeof(bfqg->blkg_path)); bic->blkcg_serial_nr = serial_nr; out: rcu_read_unlock(); @@ -640,8 +713,6 @@ static void bfq_reparent_leaf_entity(struct bfq_data *bfqd, * @bfqd: the device data structure with the root group. * @bfqg: the group to move from. * @st: the service tree with the entities. - * - * Needs queue_lock to be taken and reference to be valid over the call. */ static void bfq_reparent_active_entities(struct bfq_data *bfqd, struct bfq_group *bfqg, @@ -692,8 +763,7 @@ void bfq_pd_offline(struct blkg_policy_data *pd) /* * The idle tree may still contain bfq_queues belonging * to exited task because they never migrated to a different - * cgroup from the one being destroyed now. No one else - * can access them so it's safe to act without any lock. + * cgroup from the one being destroyed now. */ bfq_flush_idle_tree(st); |