Re: [PATCH 5/7] kernfs: Make kernfs_drain() skip draining more aggressively

Chengming Zhou
2022-08-23T05:33:31Z

On 2022/8/20 08:05, Tejun Heo wrote:
> kernfs_drain() is updated to handle whether draining is necessary or not
> rather than its caller. __kernfs_remove() now always calls kernfs_drain()
> instead of filtering based on KERNFS_ACTIVATED.
>
> kernfs_drain() now tests kn->active and kernfs_should_drain_open_files() to
> determine whether draining is necessary at all. If not, it returns %false
> without doing anything. Otherwise, it unlocks kernfs_rwsem and drains as
> before and returns %true. The return value isn't used yet.
>
> Using the precise conditions allows skipping more aggressively. This isn't a
> meaningful optimization on its own but will enable future stand-alone
> kernfs_deactivate() implementation.
>
> While at it, make minor comment updates.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> fs/kernfs/dir.c | 34 ++++++++++++++++++++--------------
> 1 file changed, 20 insertions(+), 14 deletions(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 8ae44db920d4..f857731598cd 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -460,10 +460,14 @@ void kernfs_put_active(struct kernfs_node *kn)
> * @kn: kernfs_node to drain
> *
> * Drain existing usages and nuke all existing mmaps of @kn. Mutiple
> - * removers may invoke this function concurrently on @kn and all will
> + * callers may invoke this function concurrently on @kn and all will
> * return after draining is complete.
> + *
> + * RETURNS:
> + * %false if nothing needed to be drained; otherwise, %true. On %true return,
> + * kernfs_rwsem has been released and re-acquired.
> */
> -static void kernfs_drain(struct kernfs_node *kn)
> +static bool kernfs_drain(struct kernfs_node *kn)
> __releases(&kernfs_root(kn)->kernfs_rwsem)
> __acquires(&kernfs_root(kn)->kernfs_rwsem)
> {
> @@ -472,6 +476,16 @@ static void kernfs_drain(struct kernfs_node *kn)
> lockdep_assert_held_write(&root->kernfs_rwsem);
> WARN_ON_ONCE(kernfs_active(kn));
>
> + /*
> + * Skip draining if already fully drained. This avoids draining and its
> + * lockdep annotations for nodes which have never been activated
> + * allowing embedding kernfs_remove() in create error paths without
> + * worrying about draining.
> + */
> + if (atomic_read(&kn->active) == KN_DEACTIVATED_BIAS &&
> + kernfs_should_drain_open_files(kn))

Should be !kernfs_should_drain_open_files(kn)? I see that diff is put in patch 6/7.

Thanks.

> + return false;
> +
> up_write(&root->kernfs_rwsem);
>
> if (kernfs_lockdep(kn)) {
> @@ -480,7 +494,6 @@ static void kernfs_drain(struct kernfs_node *kn)
> lock_contended(&kn->dep_map, _RET_IP_);
> }
>
> - /* but everyone should wait for draining */
> wait_event(root->deactivate_waitq,
> atomic_read(&kn->active) == KN_DEACTIVATED_BIAS);
>
> @@ -493,6 +506,8 @@ static void kernfs_drain(struct kernfs_node *kn)
> kernfs_drain_open_files(kn);
>
> down_write(&root->kernfs_rwsem);
> +
> + return true;
> }
>
> /**
> @@ -1370,23 +1385,14 @@ static void __kernfs_remove(struct kernfs_node *kn)
> pos = kernfs_leftmost_descendant(kn);
>
> /*
> - * kernfs_drain() drops kernfs_rwsem temporarily and @pos's
> + * kernfs_drain() may drop kernfs_rwsem temporarily and @pos's
> * base ref could have been put by someone else by the time
> * the function returns. Make sure it doesn't go away
> * underneath us.
> */
> kernfs_get(pos);
>
> - /*
> - * Drain iff @kn was activated. This avoids draining and
> - * its lockdep annotations for nodes which have never been
> - * activated and allows embedding kernfs_remove() in create
> - * error paths without worrying about draining.
> - */
> - if (kn->flags & KERNFS_ACTIVATED)
> - kernfs_drain(pos);
> - else
> - WARN_ON_ONCE(atomic_read(&kn->active) != KN_DEACTIVATED_BIAS);
> + kernfs_drain(pos);
>
> /*
> * kernfs_unlink_sibling() succeeds once per node. Use it