From: Boris Burkov boris@bur.io
commit 2b5463fcbdfb24e898916bcae2b1359042d26963 upstream.
Async discard does not acquire the block group reference count while it holds a reference on the discard list. This is generally OK, as the paths which destroy block groups tend to try to synchronize on cancelling async discard work. However, relying on cancelling work requires careful analysis to be sure it is safe from races with unpinning scheduling more work.
While I am unable to find a race with unpinning in the current code for either the unused bgs or relocation paths, I believe we have one in an older version of auto relocation in a Meta internal build. This suggests that this is in fact an error prone model, and could be fragile to future changes to these bg deletion paths.
To make this ownership more clear, add a refcount for async discard. If work is queued for a block group, its refcount should be incremented, and when work is completed or canceled, it should be decremented.
CC: stable@vger.kernel.org # 5.15+ Signed-off-by: Boris Burkov boris@bur.io Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/discard.c | 41 ++++++++++++++++++++++++++++++++++++++--- 1 file changed, 38 insertions(+), 3 deletions(-)
--- a/fs/btrfs/discard.c +++ b/fs/btrfs/discard.c @@ -77,6 +77,7 @@ static struct list_head *get_discard_lis static void __add_to_discard_list(struct btrfs_discard_ctl *discard_ctl, struct btrfs_block_group *block_group) { + lockdep_assert_held(&discard_ctl->lock); if (!btrfs_run_discard_work(discard_ctl)) return;
@@ -88,6 +89,8 @@ static void __add_to_discard_list(struct BTRFS_DISCARD_DELAY); block_group->discard_state = BTRFS_DISCARD_RESET_CURSOR; } + if (list_empty(&block_group->discard_list)) + btrfs_get_block_group(block_group);
list_move_tail(&block_group->discard_list, get_discard_list(discard_ctl, block_group)); @@ -107,8 +110,12 @@ static void add_to_discard_list(struct b static void add_to_discard_unused_list(struct btrfs_discard_ctl *discard_ctl, struct btrfs_block_group *block_group) { + bool queued; + spin_lock(&discard_ctl->lock);
+ queued = !list_empty(&block_group->discard_list); + if (!btrfs_run_discard_work(discard_ctl)) { spin_unlock(&discard_ctl->lock); return; @@ -120,6 +127,8 @@ static void add_to_discard_unused_list(s block_group->discard_eligible_time = (ktime_get_ns() + BTRFS_DISCARD_UNUSED_DELAY); block_group->discard_state = BTRFS_DISCARD_RESET_CURSOR; + if (!queued) + btrfs_get_block_group(block_group); list_add_tail(&block_group->discard_list, &discard_ctl->discard_list[BTRFS_DISCARD_INDEX_UNUSED]);
@@ -130,6 +139,7 @@ static bool remove_from_discard_list(str struct btrfs_block_group *block_group) { bool running = false; + bool queued = false;
spin_lock(&discard_ctl->lock);
@@ -139,7 +149,16 @@ static bool remove_from_discard_list(str }
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) + btrfs_put_block_group(block_group);
spin_unlock(&discard_ctl->lock);
@@ -212,10 +231,12 @@ again: if (block_group && now >= block_group->discard_eligible_time) { if (block_group->discard_index == BTRFS_DISCARD_INDEX_UNUSED && block_group->used != 0) { - if (btrfs_is_block_group_data_only(block_group)) + if (btrfs_is_block_group_data_only(block_group)) { __add_to_discard_list(discard_ctl, block_group); - else + } else { list_del_init(&block_group->discard_list); + btrfs_put_block_group(block_group); + } goto again; } if (block_group->discard_state == BTRFS_DISCARD_RESET_CURSOR) { @@ -502,6 +523,15 @@ static void btrfs_discard_workfn(struct 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); discard_ctl->block_group = NULL; __btrfs_discard_schedule_work(discard_ctl, now, false); spin_unlock(&discard_ctl->lock); @@ -638,8 +668,12 @@ void btrfs_discard_punt_unused_bgs_list( list_for_each_entry_safe(block_group, next, &fs_info->unused_bgs, bg_list) { list_del_init(&block_group->bg_list); - btrfs_put_block_group(block_group); btrfs_discard_queue_work(&fs_info->discard_ctl, block_group); + /* + * This put is for the get done by btrfs_mark_bg_unused. + * Queueing discard incremented it for discard's reference. + */ + btrfs_put_block_group(block_group); } spin_unlock(&fs_info->unused_bgs_lock); } @@ -669,6 +703,7 @@ static void btrfs_discard_purge_list(str if (block_group->used == 0) btrfs_mark_bg_unused(block_group); spin_lock(&discard_ctl->lock); + btrfs_put_block_group(block_group); } } spin_unlock(&discard_ctl->lock);