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    SubjectRe: [PATCH 4/7] blkcg: refactor blkcg_gq lookup and creation in blkcg_bio_issue_check()
    On Sun, Nov 12, 2017 at 02:26:10PM -0800, Tejun Heo wrote:
    > blkcg_bio_issue_check() open codes blkcg_gq lookup and creation using
    > blkg_lookup() and blkg_lookup_create(). Refactor the code so that
    >
    > * blkg_lookup_create() is renamed to blkcg_lookup_create_locked().
    >
    > * blkg_lookup_create() is now a new function which encapsulates the
    > RCU protected lookup and queue_locked protected creation.
    >
    > * blkg_lookup_create() is guaranteed to return a non-NULL blkcg_gq.
    > The NULL checks are removed from the users.
    >
    > This is pure refactoring and doesn't introduce any functional changes.
    >
    > Signed-off-by: Tejun Heo <tj@kernel.org>

    Reviewed-by: Shaohua Li <shli@kernel.org>
    > ---
    > block/blk-cgroup.c | 6 +++---
    > block/blk-throttle.c | 2 +-
    > include/linux/blk-cgroup.h | 32 +++++++++++++++++++++-----------
    > 3 files changed, 25 insertions(+), 15 deletions(-)
    >
    > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
    > index 60a4486..490b0a6 100644
    > --- a/block/blk-cgroup.c
    > +++ b/block/blk-cgroup.c
    > @@ -290,7 +290,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
    > }
    >
    > /**
    > - * blkg_lookup_create - lookup blkg, try to create one if not there
    > + * blkg_lookup_create_locked - lookup blkg, try to create one if not there
    > * @blkcg: blkcg of interest
    > * @q: request_queue of interest
    > *
    > @@ -303,8 +303,8 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
    > * value on error. If @q is dead, returns ERR_PTR(-EINVAL). If @q is not
    > * dead and bypassing, returns ERR_PTR(-EBUSY).
    > */
    > -struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
    > - struct request_queue *q)
    > +struct blkcg_gq *blkg_lookup_create_locked(struct blkcg *blkcg,
    > + struct request_queue *q)
    > {
    > struct blkcg_gq *blkg;
    >
    > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
    > index 8631763..1e6916b 100644
    > --- a/block/blk-throttle.c
    > +++ b/block/blk-throttle.c
    > @@ -2123,7 +2123,7 @@ bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg,
    > struct bio *bio)
    > {
    > struct throtl_qnode *qn = NULL;
    > - struct throtl_grp *tg = blkg_to_tg(blkg ?: q->root_blkg);
    > + struct throtl_grp *tg = blkg_to_tg(blkg);
    > struct throtl_service_queue *sq;
    > bool rw = bio_data_dir(bio);
    > bool throttled = false;
    > diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
    > index b5721d32..1de5158 100644
    > --- a/include/linux/blk-cgroup.h
    > +++ b/include/linux/blk-cgroup.h
    > @@ -172,8 +172,8 @@ extern struct cgroup_subsys_state * const blkcg_root_css;
    >
    > struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,
    > struct request_queue *q, bool update_hint);
    > -struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
    > - struct request_queue *q);
    > +struct blkcg_gq *blkg_lookup_create_locked(struct blkcg *blkcg,
    > + struct request_queue *q);
    > int blkcg_init_queue(struct request_queue *q);
    > void blkcg_drain_queue(struct request_queue *q);
    > void blkcg_exit_queue(struct request_queue *q);
    > @@ -680,6 +680,24 @@ static inline bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg
    > struct bio *bio) { return false; }
    > #endif
    >
    > +static inline struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
    > + struct request_queue *q)
    > +{
    > + struct blkcg_gq *blkg;
    > +
    > + blkg = blkg_lookup(blkcg, q);
    > + if (likely(blkg))
    > + return blkg;
    > +
    > + spin_lock_irq(q->queue_lock);
    > + blkg = blkg_lookup_create_locked(blkcg, q);
    > + spin_unlock_irq(q->queue_lock);
    > + if (likely(!IS_ERR(blkg)))
    > + return blkg;
    > + else
    > + return q->root_blkg;
    > +}
    > +
    > static inline bool blkcg_bio_issue_check(struct request_queue *q,
    > struct bio *bio)
    > {
    > @@ -693,19 +711,11 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
    > /* associate blkcg if bio hasn't attached one */
    > bio_associate_blkcg(bio, &blkcg->css);
    >
    > - blkg = blkg_lookup(blkcg, q);
    > - if (unlikely(!blkg)) {
    > - spin_lock_irq(q->queue_lock);
    > - blkg = blkg_lookup_create(blkcg, q);
    > - if (IS_ERR(blkg))
    > - blkg = NULL;
    > - spin_unlock_irq(q->queue_lock);
    > - }
    > + blkg = blkg_lookup_create(blkcg, q);
    >
    > throtl = blk_throtl_bio(q, blkg, bio);
    >
    > if (!throtl) {
    > - blkg = blkg ?: q->root_blkg;
    > blkg_rwstat_add(&blkg->stat_bytes, bio->bi_opf,
    > bio->bi_iter.bi_size);
    > blkg_rwstat_add(&blkg->stat_ios, bio->bi_opf, 1);
    > --
    > 2.9.5
    >

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