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Subject[PATCH 09/36] blkcg: move rcu_read_lock() outside of blkio_group get functions
Date
rcu_read_lock() in throtl_get_tb() and cfq_get_cfqg() holds onto
@blkcg while looking up blkg. For API cleanup, the next patch will
make the caller responsible for determining @blkcg to look blkg from
and let them specify it as a parameter. Move rcu read locking out to
the callers to prepare for the change.

-v2: Originally this patch was described as a fix for RCU read locking
bug around @blkg, which Vivek pointed out to be incorrect. It
was from misunderstanding the role of rcu locking as protecting
@blkg not @blkcg. Patch description updated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
---
block/blk-throttle.c | 18 ++++++------------
block/cfq-iosched.c | 11 +++++------
2 files changed, 11 insertions(+), 18 deletions(-)
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 3699ab4..9beaac7 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -313,25 +313,23 @@ static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
if (unlikely(blk_queue_bypass(q)))
return NULL;

- rcu_read_lock();
blkcg = task_blkio_cgroup(current);
tg = throtl_find_tg(td, blkcg);
- if (tg) {
- rcu_read_unlock();
+ if (tg)
return tg;
- }

/*
* Need to allocate a group. Allocation of group also needs allocation
* of per cpu stats which in-turn takes a mutex() and can block. Hence
* we need to drop rcu lock and queue_lock before we call alloc.
*/
- rcu_read_unlock();
spin_unlock_irq(q->queue_lock);
+ rcu_read_unlock();

tg = throtl_alloc_tg(td);

/* Group allocated and queue is still alive. take the lock */
+ rcu_read_lock();
spin_lock_irq(q->queue_lock);

/* Make sure @q is still alive */
@@ -343,7 +341,6 @@ static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
/*
* Initialize the new group. After sleeping, read the blkcg again.
*/
- rcu_read_lock();
blkcg = task_blkio_cgroup(current);

/*
@@ -354,7 +351,6 @@ static struct throtl_grp * throtl_get_tg(struct throtl_data *td)

if (__tg) {
kfree(tg);
- rcu_read_unlock();
return __tg;
}

@@ -365,7 +361,6 @@ static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
}

throtl_init_add_tg_lists(td, tg, blkcg);
- rcu_read_unlock();
return tg;
}

@@ -1150,7 +1145,6 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
* basic fields like stats and io rates. If a group has no rules,
* just update the dispatch stats in lockless manner and return.
*/
-
rcu_read_lock();
blkcg = task_blkio_cgroup(current);
tg = throtl_find_tg(td, blkcg);
@@ -1160,11 +1154,9 @@ bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
if (tg_no_rule_group(tg, rw)) {
blkiocg_update_dispatch_stats(&tg->blkg, bio->bi_size,
rw, rw_is_sync(bio->bi_rw));
- rcu_read_unlock();
- goto out;
+ goto out_unlock_rcu;
}
}
- rcu_read_unlock();

/*
* Either group has not been allocated yet or it is not an unlimited
@@ -1222,6 +1214,8 @@ queue_bio:

out_unlock:
spin_unlock_irq(q->queue_lock);
+out_unlock_rcu:
+ rcu_read_unlock();
out:
return throttled;
}
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 61693d3..6063c44 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1128,13 +1128,10 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
struct cfq_group *cfqg = NULL, *__cfqg = NULL;
struct request_queue *q = cfqd->queue;

- rcu_read_lock();
blkcg = task_blkio_cgroup(current);
cfqg = cfq_find_cfqg(cfqd, blkcg);
- if (cfqg) {
- rcu_read_unlock();
+ if (cfqg)
return cfqg;
- }

/*
* Need to allocate a group. Allocation of group also needs allocation
@@ -1164,7 +1161,6 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)

if (__cfqg) {
kfree(cfqg);
- rcu_read_unlock();
return __cfqg;
}

@@ -1172,7 +1168,6 @@ static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd)
cfqg = &cfqd->root_group;

cfq_init_add_cfqg_lists(cfqd, cfqg, blkcg);
- rcu_read_unlock();
return cfqg;
}

@@ -2870,6 +2865,8 @@ cfq_find_alloc_queue(struct cfq_data *cfqd, bool is_sync,
struct cfq_group *cfqg;

retry:
+ rcu_read_lock();
+
cfqg = cfq_get_cfqg(cfqd);
cic = cfq_cic_lookup(cfqd, ioc);
/* cic always exists here */
@@ -2885,6 +2882,7 @@ retry:
cfqq = new_cfqq;
new_cfqq = NULL;
} else if (gfp_mask & __GFP_WAIT) {
+ rcu_read_unlock();
spin_unlock_irq(cfqd->queue->queue_lock);
new_cfqq = kmem_cache_alloc_node(cfq_pool,
gfp_mask | __GFP_ZERO,
@@ -2910,6 +2908,7 @@ retry:
if (new_cfqq)
kmem_cache_free(cfq_pool, new_cfqq);

+ rcu_read_unlock();
return cfqq;
}

--
1.7.7.3


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