Re: [PATCH] btrfs: avoid taking the device_list_mutex in btrfs_run_dev_stats()

[David Sterba <dsterba@suse.cz>]

On Wed, Mar 18, 2026 at 01:39:51PM +0000, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> btrfs_run_dev_stats() is called during the critical section of a
> transaction commit and it takes the device_list_mutex, which is also
> acquired by fitrim, which does discard operations while holding that
> mutex. Most of the time, if we are on a healthy filesystem, we don't have
> new stat updates to persist in the device tree, so blocking on the
> device_list_mutex is just wasting time and making any tasks that need to
> start a new transaction wait longer that necessary.
> 
> Since the device list is RCU safe/protected, make btrfs_run_dev_stats()
> do an initial check for device stat updates using RCU and quit without
> taking the device_list_mutex in case there are no new device stats that
> need to be persisted in the device tree.
> 
> Also note that adding/removing devices also requires starting a
> transaction, and since btrfs_run_dev_stats() is called from the critical
> section of a transaction commit, no one can be concurrently adding or
> removing a device while btrfs_run_dev_stats() is called.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: David Sterba <dsterba@suse.com>