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Date: Mon, 17 Dec 2012 14:28:08 -0500
From: Vivek Goyal <vgoyal@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: lizefan@...wei.com, axboe@...nel.dk,
containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
linux-kernel@...r.kernel.org, ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCH 02/12] blkcg: reorganize blkg_lookup_create() and friends
On Fri, Dec 14, 2012 at 02:41:15PM -0800, Tejun Heo wrote:
> Reorganize such that
>
> * __blkg_lookup() takes bool param @update_hint to determine whether
> to update hint.
>
> * __blkg_lookup_create() no longer performs lookup before trying to
> create. Renamed to blkg_create().
>
> * blkg_lookup_create() now performs lookup and then invokes
> blkg_create() if lookup fails.
>
> * root_blkg creation in blkcg_activate_policy() updated accordingly.
> Note that blkcg_activate_policy() no longer updates lookup hint if
> root_blkg already exists.
>
> Except for the last lookup hint bit which is immaterial, this is pure
> reorganization and doesn't introduce any visible behavior change.
> This is to prepare for proper hierarchy support.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
looks good to me. I particularly like cleanup of __blkg_lookup_create()
which freed new_blkg if a blkg was already found.
Acked-by: Vivek Goyal <vgoyal@...hat.com>
Vivek
> ---
> block/blk-cgroup.c | 75 +++++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 52 insertions(+), 23 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index feda49f..ffbd237 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -126,7 +126,7 @@ err_free:
> }
>
> static struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg,
> - struct request_queue *q)
> + struct request_queue *q, bool update_hint)
> {
> struct blkcg_gq *blkg;
>
> @@ -135,14 +135,19 @@ static struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg,
> return blkg;
>
> /*
> - * Hint didn't match. Look up from the radix tree. Note that we
> - * may not be holding queue_lock and thus are not sure whether
> - * @blkg from blkg_tree has already been removed or not, so we
> - * can't update hint to the lookup result. Leave it to the caller.
> + * Hint didn't match. Look up from the radix tree. Note that the
> + * hint can only be updated under queue_lock as otherwise @blkg
> + * could have already been removed from blkg_tree. The caller is
> + * responsible for grabbing queue_lock if @update_hint.
> */
> blkg = radix_tree_lookup(&blkcg->blkg_tree, q->id);
> - if (blkg && blkg->q == q)
> + if (blkg && blkg->q == q) {
> + if (update_hint) {
> + lockdep_assert_held(q->queue_lock);
> + rcu_assign_pointer(blkcg->blkg_hint, blkg);
> + }
> return blkg;
> + }
>
> return NULL;
> }
> @@ -162,7 +167,7 @@ struct blkcg_gq *blkg_lookup(struct blkcg *blkcg, struct request_queue *q)
>
> if (unlikely(blk_queue_bypass(q)))
> return NULL;
> - return __blkg_lookup(blkcg, q);
> + return __blkg_lookup(blkcg, q, false);
> }
> EXPORT_SYMBOL_GPL(blkg_lookup);
>
> @@ -170,9 +175,9 @@ EXPORT_SYMBOL_GPL(blkg_lookup);
> * If @new_blkg is %NULL, this function tries to allocate a new one as
> * necessary using %GFP_ATOMIC. @new_blkg is always consumed on return.
> */
> -static struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
> - struct request_queue *q,
> - struct blkcg_gq *new_blkg)
> +static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
> + struct request_queue *q,
> + struct blkcg_gq *new_blkg)
> {
> struct blkcg_gq *blkg;
> int ret;
> @@ -180,13 +185,6 @@ static struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
> WARN_ON_ONCE(!rcu_read_lock_held());
> lockdep_assert_held(q->queue_lock);
>
> - /* lookup and update hint on success, see __blkg_lookup() for details */
> - blkg = __blkg_lookup(blkcg, q);
> - if (blkg) {
> - rcu_assign_pointer(blkcg->blkg_hint, blkg);
> - goto out_free;
> - }
> -
> /* blkg holds a reference to blkcg */
> if (!css_tryget(&blkcg->css)) {
> blkg = ERR_PTR(-EINVAL);
> @@ -223,16 +221,39 @@ out_free:
> return blkg;
> }
>
> +/**
> + * blkg_lookup_create - lookup blkg, try to create one if not there
> + * @blkcg: blkcg of interest
> + * @q: request_queue of interest
> + *
> + * Lookup blkg for the @blkcg - @q pair. If it doesn't exist, try to
> + * create one. This function should be called under RCU read lock and
> + * @q->queue_lock.
> + *
> + * Returns pointer to the looked up or created blkg on success, ERR_PTR()
> + * 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;
> +
> + WARN_ON_ONCE(!rcu_read_lock_held());
> + lockdep_assert_held(q->queue_lock);
> +
> /*
> * This could be the first entry point of blkcg implementation and
> * we shouldn't allow anything to go through for a bypassing queue.
> */
> if (unlikely(blk_queue_bypass(q)))
> return ERR_PTR(blk_queue_dead(q) ? -EINVAL : -EBUSY);
> - return __blkg_lookup_create(blkcg, q, NULL);
> +
> + blkg = __blkg_lookup(blkcg, q, true);
> + if (blkg)
> + return blkg;
> +
> + return blkg_create(blkcg, q, NULL);
> }
> EXPORT_SYMBOL_GPL(blkg_lookup_create);
>
> @@ -777,7 +798,7 @@ int blkcg_activate_policy(struct request_queue *q,
> const struct blkcg_policy *pol)
> {
> LIST_HEAD(pds);
> - struct blkcg_gq *blkg;
> + struct blkcg_gq *blkg, *new_blkg;
> struct blkg_policy_data *pd, *n;
> int cnt = 0, ret;
> bool preloaded;
> @@ -786,19 +807,27 @@ int blkcg_activate_policy(struct request_queue *q,
> return 0;
>
> /* preallocations for root blkg */
> - blkg = blkg_alloc(&blkcg_root, q, GFP_KERNEL);
> - if (!blkg)
> + new_blkg = blkg_alloc(&blkcg_root, q, GFP_KERNEL);
> + if (!new_blkg)
> return -ENOMEM;
>
> preloaded = !radix_tree_preload(GFP_KERNEL);
>
> blk_queue_bypass_start(q);
>
> - /* make sure the root blkg exists and count the existing blkgs */
> + /*
> + * Make sure the root blkg exists and count the existing blkgs. As
> + * @q is bypassing at this point, blkg_lookup_create() can't be
> + * used. Open code it.
> + */
> spin_lock_irq(q->queue_lock);
>
> rcu_read_lock();
> - blkg = __blkg_lookup_create(&blkcg_root, q, blkg);
> + blkg = __blkg_lookup(&blkcg_root, q, false);
> + if (blkg)
> + blkg_free(new_blkg);
> + else
> + blkg = blkg_create(&blkcg_root, q, new_blkg);
> rcu_read_unlock();
>
> if (preloaded)
> --
> 1.7.11.7
--
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