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btrfs: make btrfs_discard_workfn() block_group ref explicit

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[ 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>
This commit is contained in:
Boris Burkov 2025-03-03 15:01:05 -08:00 committed by Greg Kroah-Hartman
parent a4666a8127
commit 4e74f91a6f
1 changed files with 16 additions and 18 deletions
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34
fs/btrfs/discard.c
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@ -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 @@ again:
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);