Re: [PATCH 1/2] btrfs: zoned: use rcu list for iterating devices to collect stats

[David Sterba]

On Tue, Jan 23, 2024 at 07:49:22AM +0000, Johannes Thumshirn wrote:
> On 22.01.24 22:35, David Sterba wrote:
> > On Mon, Jan 22, 2024 at 02:51:03AM -0800, Johannes Thumshirn wrote:
> >> As btrfs_zoned_should_reclaim only has to iterate the device list in order
> >> to collect stats on the device's total and used bytes, we don't need to
> >> take the full blown mutex, but can iterate the device list in a rcu_read
> >> context.
> >>
> >> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> >> ---
> >>   fs/btrfs/zoned.c | 6 +++---
> >>   1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> >> index 168af9d000d1..b7e7b5a5a6fa 100644
> >> --- a/fs/btrfs/zoned.c
> >> +++ b/fs/btrfs/zoned.c
> >> @@ -2423,15 +2423,15 @@ 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);
> >> -	list_for_each_entry(device, &fs_devices->devices, dev_list) {
> >> +	rcu_read_lock();
> >> +	list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {
> >>   		if (!device->bdev)
> >>   			continue;
> >>   
> >>   		total += device->disk_total_bytes;
> >>   		used += device->bytes_used;
> >>   	}
> >> -	mutex_unlock(&fs_devices->device_list_mutex);
> >> +	rcu_read_unlock();
> > 
> > This is basically only a hint and inaccuracies in the total or used
> > values would be transient, right? The sum is calculated each time the
> > funciton is called, not stored anywhere so in the unlikely case of
> > device removal it may skip reclaim once, but then pick it up later.
> > Any actual removal of the block groups in verified again and properly
> > locked in btrfs_reclaim_bgs_work().
> > 
> 
> Yes.

So please add it to the changelog as an explanation why the mutex -> rcu
switch is safe, thanks.