2019-11-05 16:11:48

by Tejun Heo

Subject: [PATCH block/for-5.4-fixes] blkcg: make blkcg_print_stat() print stats only for online blkgs

blkcg_print_stat() iterates blkgs under RCU and doesn't test whether
the blkg is online. This can call into pd_stat_fn() on a pd which is
still being initialized leading to an oops.

The heaviest operation - recursively summing up rwstat counters - is
already done while holding the queue_lock. Expand queue_lock to cover
the other operations and skip the blkg if it isn't online yet. The
online state is protected by both blkcg and queue locks, so this
guarantees that only online blkgs are processed.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Roman Gushchin <[email protected]>
Cc: Josef Bacik <[email protected]>
Fixes: 903d23f0a354 ("blk-cgroup: allow controllers to output their own stats")
Cc: [email protected] # v4.19+
---
block/blk-cgroup.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -934,9 +934,14 @@ static int blkcg_print_stat(struct seq_f
int i;
bool has_stats = false;

+ spin_lock_irq(&blkg->q->queue_lock);
+
+ if (!blkg->online)
+ goto skip;
+
dname = blkg_dev_name(blkg);
if (!dname)
- continue;
+ goto skip;

/*
* Hooray string manipulation, count is the size written NOT
@@ -946,8 +951,6 @@ static int blkcg_print_stat(struct seq_f
*/
off += scnprintf(buf+off, size-off, "%s ", dname);

- spin_lock_irq(&blkg->q->queue_lock);
-
blkg_rwstat_recursive_sum(blkg, NULL,
offsetof(struct blkcg_gq, stat_bytes), &rwstat);
rbytes = rwstat.cnt[BLKG_RWSTAT_READ];
@@ -960,8 +963,6 @@ static int blkcg_print_stat(struct seq_f
wios = rwstat.cnt[BLKG_RWSTAT_WRITE];
dios = rwstat.cnt[BLKG_RWSTAT_DISCARD];

- spin_unlock_irq(&blkg->q->queue_lock);
-
if (rbytes || wbytes || rios || wios) {
has_stats = true;
off += scnprintf(buf+off, size-off,
@@ -999,6 +1000,8 @@ static int blkcg_print_stat(struct seq_f
seq_commit(sf, -1);
}
}
+ skip:
+ spin_unlock_irq(&blkg->q->queue_lock);
}

rcu_read_unlock();

2019-11-07 00:10:27

by Jens Axboe

Subject: Re: [PATCH block/for-5.4-fixes] blkcg: make blkcg_print_stat() print stats only for online blkgs

On 11/5/19 9:09 AM, Tejun Heo wrote:
> blkcg_print_stat() iterates blkgs under RCU and doesn't test whether
> the blkg is online. This can call into pd_stat_fn() on a pd which is
> still being initialized leading to an oops.
>
> The heaviest operation - recursively summing up rwstat counters - is
> already done while holding the queue_lock. Expand queue_lock to cover
> the other operations and skip the blkg if it isn't online yet. The
> online state is protected by both blkcg and queue locks, so this
> guarantees that only online blkgs are processed.

Applied, thanks Tejun.

--
Jens Axboe