On Thu, Feb 06, 2025 at 04:42:51PM +1100, NeilBrown wrote:
> vfs_rmdir takes an exclusive lock on the target directory to ensure
> nothing new is created in it while the rmdir progresses. With the
It also excludes concurrent mount operations.
> possibility of async updates continuing after the inode lock is dropped
> we now need extra protection.
>
> Any async updates will have DCACHE_PAR_UPDATE set on the dentry. We
> simply wait for that flag to be cleared on all children.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> fs/dcache.c | 2 +-
> fs/namei.c | 40 ++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index fb331596f1b1..90dee859d138 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -53,7 +53,7 @@
> * - d_lru
> * - d_count
> * - d_unhashed()
> - * - d_parent and d_chilren
> + * - d_parent and d_children
> * - childrens' d_sib and d_parent
> * - d_u.d_alias, d_inode
> *
> diff --git a/fs/namei.c b/fs/namei.c
> index 3a107d6098be..e8a85c9f431c 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1839,6 +1839,27 @@ bool d_update_lock(struct dentry *dentry,
> return true;
> }
>
> +static void d_update_wait(struct dentry *dentry, unsigned int subclass)
> +{
> + /* Note this may only ever be called in a context where we have
> + * a lock preventing this dentry from becoming locked, possibly
> + * an update lock on the parent dentry. The must be a smp_mb()
> + * after that lock is taken and before this is called so that
> + * the following test is safe. d_update_lock() provides that
> + * barrier.
> + */
> + if (!(dentry->d_flags & DCACHE_PAR_UPDATE))
> + return
> + lock_acquire_exclusive(&dentry->d_update_map, subclass,
> + 0, NULL, _THIS_IP_);
> + spin_lock(&dentry->d_lock);
> + wait_var_event_spinlock(&dentry->d_flags,
> + !check_dentry_locked(dentry),
> + &dentry->d_lock);
> + spin_unlock(&dentry->d_lock);
> + lock_map_release(&dentry->d_update_map);
> +}
> +
> bool d_update_trylock(struct dentry *dentry,
> struct dentry *base,
> const struct qstr *last)
> @@ -4688,6 +4709,7 @@ int vfs_rmdir(struct mnt_idmap *idmap, struct inode *dir,
> struct dentry *dentry)
> {
> int error = may_delete(idmap, dir, dentry, 1);
> + struct dentry *child;
>
> if (error)
> return error;
> @@ -4697,6 +4719,24 @@ int vfs_rmdir(struct mnt_idmap *idmap, struct inode *dir,
>
> dget(dentry);
> inode_lock(dentry->d_inode);
> + /*
> + * Some children of dentry might be active in an async update.
> + * We need to wait for them. New children cannot be locked
> + * while the inode lock is held.
> + */
> +again:
> + spin_lock(&dentry->d_lock);
> + for (child = d_first_child(dentry); child;
> + child = d_next_sibling(child)) {
> + if (child->d_flags & DCACHE_PAR_UPDATE) {
> + dget(child);
> + spin_unlock(&dentry->d_lock);
> + d_update_wait(child, I_MUTEX_CHILD);
> + dput(child);
> + goto again;
> + }
> + }
> + spin_unlock(&dentry->d_lock);
That looks like it can cause stalls when you call rmdir on a directory
that has a lots of children and a larg-ish subset of them has pending
async updates, no?
