[2/3] btrfs: get used bytes while holding lock at btrfs_reclaim_bgs_work()

Commit Message

Filipe Manana Feb. 25, 2025, 10:57 a.m. UTC

From: Filipe Manana <[e-mail]>

At btrfs_reclaim_bgs_work(), we are grabbing twice the used bytes counter
of the block group while not holding the block group's spinlock. This can
result in races, reported by KCSAN and similar tools, since a concurrent
task can be updating that counter while at btrfs_update_block_group().

So avoid these races by grabbing the counter in the critical section right
above that is delimited by the block group's spinlock. This also avoids
using two different values of the counter in case it changes in between
each read. This silences KCSAN and is required for the next patch in the
series too.

Fixes: 243192b67649 ("btrfs: report reclaim stats in sysfs")
Signed-off-by: Filipe Manana <[e-mail]>
---
 fs/btrfs/block-group.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

Patch

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 2e174c14ca0a..1200c5efeb3d 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1823,7 +1823,7 @@  void btrfs_reclaim_bgs_work(struct work_struct *work)
 	list_sort(NULL, &fs_info->reclaim_bgs, reclaim_bgs_cmp);
 	while (!list_empty(&fs_info->reclaim_bgs)) {
 		u64 zone_unusable;
-		u64 reclaimed;
+		u64 used;
 		int ret = 0;
 
 		bg = list_first_entry(&fs_info->reclaim_bgs,
@@ -1919,19 +1919,30 @@  void btrfs_reclaim_bgs_work(struct work_struct *work)
 		if (ret < 0)
 			goto next;
 
+		/*
+		 * Grab the used bytes counter while holding the block groups's
+		 * spinlock to prevent races with tasks concurrently updating it
+		 * due to extent allocation and deallocation (running
+		 * btrfs_update_block_group()) - we have set the block group to
+		 * RO but that only prevents extent reservation, allocation
+		 * happens after reservation.
+		 */
+		spin_lock(&bg->lock);
+		used = bg->used;
+		spin_unlock(&bg->lock);
+
 		btrfs_info(fs_info,
 			"reclaiming chunk %llu with %llu%% used %llu%% unusable",
 				bg->start,
-				div64_u64(bg->used * 100, bg->length),
+				div64_u64(used * 100, bg->length),
 				div64_u64(zone_unusable * 100, bg->length));
 		trace_btrfs_reclaim_block_group(bg);
-		reclaimed = bg->used;
 		ret = btrfs_relocate_chunk(fs_info, bg->start);
 		if (ret) {
 			btrfs_dec_block_group_ro(bg);
 			btrfs_err(fs_info, "error relocating chunk %llu",
 				  bg->start);
-			reclaimed = 0;
+			used = 0;
 			spin_lock(&space_info->lock);
 			space_info->reclaim_errors++;
 			if (READ_ONCE(space_info->periodic_reclaim))
@@ -1940,7 +1951,7 @@  void btrfs_reclaim_bgs_work(struct work_struct *work)
 		}
 		spin_lock(&space_info->lock);
 		space_info->reclaim_count++;
-		space_info->reclaim_bytes += reclaimed;
+		space_info->reclaim_bytes += used;
 		spin_unlock(&space_info->lock);
 
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