Commit 27e6fa99 authored by Dennis Zhou (Facebook)'s avatar Dennis Zhou (Facebook) Committed by Jens Axboe

blkcg: fix ref count issue with bio_blkcg using task_css

The accessor function bio_blkcg either returns the blkcg associated with
the bio or finds one in the current context. This can cause an issue
when trying to associate a bio with a blkcg. Particularly, it's the
third case that is problematic:

	return css_to_blkcg(task_css(current, io_cgrp_id));

As the above may race against task migration and the cgroup exiting, it
is not always ok to take a reference on the blkcg returned from
bio_blkcg.

This patch adds association ahead of calling bio_blkcg rather than
after. This makes association a required and explicit step along the
code paths for calling bio_blkcg. blk_get_rl is modified as well to get
a reference to the blkcg it may use and blk_put_rl will always put the
reference back. Association is also moved above the bio_blkcg call to
ensure it will not return NULL in blk-iolatency.

BFQ and CFQ utilize this flaw, but due to the complexity, I do not want
to address this in this series. I've created a private version of the
function with notes not to use it describing the flaw. Hopefully soon,
that code can be cleaned up.
Signed-off-by: default avatarDennis Zhou <dennisszhou@gmail.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 9ff01255
......@@ -642,7 +642,7 @@ void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio)
uint64_t serial_nr;
rcu_read_lock();
serial_nr = bio_blkcg(bio)->css.serial_nr;
serial_nr = __bio_blkcg(bio)->css.serial_nr;
/*
* Check whether blkcg has changed. The condition may trigger
......@@ -651,7 +651,7 @@ void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio)
if (unlikely(!bfqd) || likely(bic->blkcg_serial_nr == serial_nr))
goto out;
bfqg = __bfq_bic_change_cgroup(bfqd, bic, bio_blkcg(bio));
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
......
......@@ -4359,7 +4359,7 @@ static struct bfq_queue *bfq_get_queue(struct bfq_data *bfqd,
rcu_read_lock();
bfqg = bfq_find_set_group(bfqd, bio_blkcg(bio));
bfqg = bfq_find_set_group(bfqd, __bio_blkcg(bio));
if (!bfqg) {
bfqq = &bfqd->oom_bfqq;
goto out;
......
......@@ -1988,13 +1988,19 @@ int bio_associate_blkcg_from_page(struct bio *bio, struct page *page)
*
* This function takes an extra reference of @blkcg_css which will be put
* when @bio is released. The caller must own @bio and is responsible for
* synchronizing calls to this function.
* synchronizing calls to this function. If @blkcg_css is NULL, a call to
* blkcg_get_css finds the current css from the kthread or task.
*/
int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css)
{
if (unlikely(bio->bi_css))
return -EBUSY;
css_get(blkcg_css);
if (blkcg_css)
css_get(blkcg_css);
else
blkcg_css = blkcg_get_css();
bio->bi_css = blkcg_css;
return 0;
}
......
......@@ -401,8 +401,8 @@ static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio,
return;
rcu_read_lock();
bio_associate_blkcg(bio, NULL);
blkcg = bio_blkcg(bio);
bio_associate_blkcg(bio, &blkcg->css);
blkg = blkg_lookup(blkcg, q);
if (unlikely(!blkg)) {
if (!lock)
......
......@@ -3753,7 +3753,7 @@ static void check_blkcg_changed(struct cfq_io_cq *cic, struct bio *bio)
uint64_t serial_nr;
rcu_read_lock();
serial_nr = bio_blkcg(bio)->css.serial_nr;
serial_nr = __bio_blkcg(bio)->css.serial_nr;
rcu_read_unlock();
/*
......@@ -3818,7 +3818,7 @@ cfq_get_queue(struct cfq_data *cfqd, bool is_sync, struct cfq_io_cq *cic,
struct cfq_group *cfqg;
rcu_read_lock();
cfqg = cfq_lookup_cfqg(cfqd, bio_blkcg(bio));
cfqg = cfq_lookup_cfqg(cfqd, __bio_blkcg(bio));
if (!cfqg) {
cfqq = &cfqd->oom_cfqq;
goto out;
......
......@@ -230,22 +230,100 @@ 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);
/**
* blkcg_css - find the current css
*
* Find the css associated with either the kthread or the current task.
* This may return a dying css, so it is up to the caller to use tryget logic
* to confirm it is alive and well.
*/
static inline struct cgroup_subsys_state *blkcg_css(void)
{
struct cgroup_subsys_state *css;
css = kthread_blkcg();
if (css)
return css;
return task_css(current, io_cgrp_id);
}
/**
* blkcg_get_css - find and get a reference to the css
*
* Find the css associated with either the kthread or the current task.
* This takes a reference on the blkcg which will need to be managed by the
* caller.
*/
static inline struct cgroup_subsys_state *blkcg_get_css(void)
{
struct cgroup_subsys_state *css;
rcu_read_lock();
css = kthread_blkcg();
if (css) {
css_get(css);
} else {
/*
* This is a bit complicated. It is possible task_css is seeing
* an old css pointer here. This is caused by the current
* thread migrating away from this cgroup and this cgroup dying.
* css_tryget() will fail when trying to take a ref on a cgroup
* that's ref count has hit 0.
*
* Therefore, if it does fail, this means current must have
* been swapped away already and this is waiting for it to
* propagate on the polling cpu. Hence the use of cpu_relax().
*/
while (true) {
css = task_css(current, io_cgrp_id);
if (likely(css_tryget(css)))
break;
cpu_relax();
}
}
rcu_read_unlock();
return css;
}
static inline struct blkcg *css_to_blkcg(struct cgroup_subsys_state *css)
{
return css ? container_of(css, struct blkcg, css) : NULL;
}
static inline struct blkcg *bio_blkcg(struct bio *bio)
/**
* __bio_blkcg - internal version of bio_blkcg for bfq and cfq
*
* DO NOT USE.
* There is a flaw using this version of the function. In particular, this was
* used in a broken paradigm where association was called on the given css. It
* is possible though that the returned css from task_css() is in the process
* of dying due to migration of the current task. So it is improper to assume
* *_get() is going to succeed. Both BFQ and CFQ rely on this logic and will
* take additional work to handle more gracefully.
*/
static inline struct blkcg *__bio_blkcg(struct bio *bio)
{
struct cgroup_subsys_state *css;
if (bio && bio->bi_css)
return css_to_blkcg(bio->bi_css);
return css_to_blkcg(blkcg_css());
}
/**
* bio_blkcg - grab the blkcg associated with a bio
* @bio: target bio
*
* This returns the blkcg associated with a bio, NULL if not associated.
* Callers are expected to either handle NULL or know association has been
* done prior to calling this.
*/
static inline struct blkcg *bio_blkcg(struct bio *bio)
{
if (bio && bio->bi_css)
return css_to_blkcg(bio->bi_css);
css = kthread_blkcg();
if (css)
return css_to_blkcg(css);
return css_to_blkcg(task_css(current, io_cgrp_id));
return NULL;
}
static inline bool blk_cgroup_congested(void)
......@@ -534,6 +612,10 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
rcu_read_lock();
blkcg = bio_blkcg(bio);
if (blkcg)
css_get(&blkcg->css);
else
blkcg = css_to_blkcg(blkcg_get_css());
/* bypass blkg lookup and use @q->root_rl directly for root */
if (blkcg == &blkcg_root)
......@@ -565,6 +647,8 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
*/
static inline void blk_put_rl(struct request_list *rl)
{
/* an additional ref is always taken for rl */
css_put(&rl->blkg->blkcg->css);
if (rl->blkg->blkcg != &blkcg_root)
blkg_put(rl->blkg);
}
......@@ -805,10 +889,10 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
bool throtl = false;
rcu_read_lock();
blkcg = bio_blkcg(bio);
/* associate blkcg if bio hasn't attached one */
bio_associate_blkcg(bio, &blkcg->css);
bio_associate_blkcg(bio, NULL);
blkcg = bio_blkcg(bio);
blkg = blkg_lookup(blkcg, q);
if (unlikely(!blkg)) {
......@@ -930,6 +1014,7 @@ static inline int blkcg_activate_policy(struct request_queue *q,
static inline void blkcg_deactivate_policy(struct request_queue *q,
const struct blkcg_policy *pol) { }
static inline struct blkcg *__bio_blkcg(struct bio *bio) { return NULL; }
static inline struct blkcg *bio_blkcg(struct bio *bio) { return NULL; }
static inline struct blkg_policy_data *blkg_to_pd(struct blkcg_gq *blkg,
......
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