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From: Johannes Thumshirn <jth@kernel.org>
Date: Thu, 28 Mar 2024 14:56:31 +0100
Subject: [PATCH RFC PATCH 1/3] btrfs: zoned: traverse device list in should
 reclaim under rcu_read_lock
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Message-Id: <20240328-hans-v1-1-4cd558959407@kernel.org>
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To: Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>, 
 David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, 
 Hans Holmberg <Hans.Holmberg@wdc.com>, Naohiro Aota <Naohiro.Aota@wdc.com>, 
 hch@lst.de, Damien LeMoal <dlemoal@kernel.org>, Boris Burkov <boris@bur.io>, 
 Johannes Thumshirn <johannes.thumshirn@wdc.com>

From: Johannes Thumshirn <johannes.thumshirn@wdc.com>

As btrfs_zoned_should_reclaim() traverses the device list with the
device_list_mutex held. But we're never changing the device list. All we
do is gathering the used and total bytes.

So change the list traversal from the holding the device_list_mutex to
rcu_read_lock(). This also opens up the possibilities to call
btrfs_zoned_should_reclaim() with the chunk_mutex held.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 fs/btrfs/zoned.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
index 4cba80b34387..d51faf7f4162 100644
--- a/fs/btrfs/zoned.c
+++ b/fs/btrfs/zoned.c
@@ -2446,7 +2446,7 @@ bool btrfs_zoned_should_reclaim(struct btrfs_fs_info *fs_info)
 	if (fs_info->bg_reclaim_threshold == 0)
 		return false;
 
-	mutex_lock(&fs_devices->device_list_mutex);
+	rcu_read_lock();
 	list_for_each_entry(device, &fs_devices->devices, dev_list) {
 		if (!device->bdev)
 			continue;
@@ -2454,7 +2454,7 @@ bool btrfs_zoned_should_reclaim(struct btrfs_fs_info *fs_info)
 		total += device->disk_total_bytes;
 		used += device->bytes_used;
 	}
-	mutex_unlock(&fs_devices->device_list_mutex);
+	rcu_read_unlock();
 
 	factor = div64_u64(used * 100, total);
 	return factor >= fs_info->bg_reclaim_threshold;

-- 
2.35.3