[PATCH AUTOSEL 6.6 044/294] btrfs: make btrfs_discard_workfn() block_group ref explicit

Sasha Levin
2025-05-05T22:58:00Z

From: Boris Burkov <boris@bur.io>

[ Upstream commit 895c6721d310c036dcfebb5ab845822229fa35eb ]

Currently, the async discard machinery owns a ref to the block_group
when the block_group is queued on a discard list. However, to handle
races with discard cancellation and the discard workfn, we have a
specific logic to detect that the block_group is *currently* running in
the workfn, to protect the workfn's usage amidst cancellation.

As far as I can tell, this doesn't have any overt bugs (though
finish_discard_pass() and remove_from_discard_list() racing can have a
surprising outcome for the caller of remove_from_discard_list() in that
it is again added at the end).

But it is needlessly complicated to rely on locking and the nullity of
discard_ctl->block_group. Simplify this significantly by just taking a
refcount while we are in the workfn and unconditionally drop it in both
the remove and workfn paths, regardless of if they race.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/discard.c | 34 ++++++++++++++++------------------
1 file changed, 16 insertions(+), 18 deletions(-)

diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c
index 944a7340f6a44..3981c941f5b55 100644
--- a/fs/btrfs/discard.c
+++ b/fs/btrfs/discard.c
@@ -167,13 +167,7 @@ static bool remove_from_discard_list(struct btrfs_discard_ctl *discard_ctl,
block_group->discard_eligible_time = 0;
queued = !list_empty(&block_group->discard_list);
list_del_init(&block_group->discard_list);
- /*
- * If the block group is currently running in the discard workfn, we
- * don't want to deref it, since it's still being used by the workfn.
- * The workfn will notice this case and deref the block group when it is
- * finished.
- */
- if (queued && !running)
+ if (queued)
btrfs_put_block_group(block_group);

spin_unlock(&discard_ctl->lock);
@@ -260,9 +254,10 @@ static struct btrfs_block_group *peek_discard_list(
block_group->discard_cursor = block_group->start;
block_group->discard_state = BTRFS_DISCARD_EXTENTS;
}
- discard_ctl->block_group = block_group;
}
if (block_group) {
+ btrfs_get_block_group(block_group);
+ discard_ctl->block_group = block_group;
*discard_state = block_group->discard_state;
*discard_index = block_group->discard_index;
}
@@ -493,9 +488,20 @@ static void btrfs_discard_workfn(struct work_struct *work)

block_group = peek_discard_list(discard_ctl, &discard_state,
&discard_index, now);
- if (!block_group || !btrfs_run_discard_work(discard_ctl))
+ if (!block_group)
return;
+ if (!btrfs_run_discard_work(discard_ctl)) {
+ spin_lock(&discard_ctl->lock);
+ btrfs_put_block_group(block_group);
+ discard_ctl->block_group = NULL;
+ spin_unlock(&discard_ctl->lock);
+ return;
+ }
if (now < block_group->discard_eligible_time) {
+ spin_lock(&discard_ctl->lock);
+ btrfs_put_block_group(block_group);
+ discard_ctl->block_group = NULL;
+ spin_unlock(&discard_ctl->lock);
btrfs_discard_schedule_work(discard_ctl, false);
return;
}
@@ -547,15 +553,7 @@ static void btrfs_discard_workfn(struct work_struct *work)
spin_lock(&discard_ctl->lock);
discard_ctl->prev_discard = trimmed;
discard_ctl->prev_discard_time = now;
- /*
- * If the block group was removed from the discard list while it was
- * running in this workfn, then we didn't deref it, since this function
- * still owned that reference. But we set the discard_ctl->block_group
- * back to NULL, so we can use that condition to know that now we need
- * to deref the block_group.
- */
- if (discard_ctl->block_group == NULL)
- btrfs_put_block_group(block_group);
+ btrfs_put_block_group(block_group);
discard_ctl->block_group = NULL;
__btrfs_discard_schedule_work(discard_ctl, now, false);
spin_unlock(&discard_ctl->lock);
--
2.39.5