release/5.2.28/blkcg-make-blkcg_print_stat-print-stats-only-for-onl.patch

From 79dfd0a06a59295a9e093f0c7b5ffeb9d55d4908 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Tue, 5 Nov 2019 08:09:51 -0800
Subject: [PATCH] blkcg: make blkcg_print_stat() print stats only for online
 blkgs

commit b0814361a25cba73a224548843ed92d8ea78715a upstream.

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 <tj@kernel.org>
Reported-by: Roman Gushchin <guro@fb.com>
Cc: Josef Bacik <jbacik@fb.com>
Fixes: 903d23f0a354 ("blk-cgroup: allow controllers to output their own stats")
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index e4715b35d42c..a28865838233 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -945,9 +945,14 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 		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
@@ -957,8 +962,6 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 		 */
 		off += scnprintf(buf+off, size-off, "%s ", dname);
 
-		spin_lock_irq(&blkg->q->queue_lock);
-
 		rwstat = blkg_rwstat_recursive_sum(blkg, NULL,
 					offsetof(struct blkcg_gq, stat_bytes));
 		rbytes = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_READ]);
@@ -971,8 +974,6 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 		wios = atomic64_read(&rwstat.aux_cnt[BLKG_RWSTAT_WRITE]);
 		dios = atomic64_read(&rwstat.aux_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,
@@ -1013,6 +1014,8 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
 				seq_commit(sf, -1);
 			}
 		}
+	skip:
+		spin_unlock_irq(&blkg->q->queue_lock);
 	}
 
 	rcu_read_unlock();
-- 
2.7.4