Re: [RFC PATCH v2 1/8] Use atomic type for ucounts reference counting

Eric W. Biederman
2021-01-13T16:32:54Z

Alexey Gladkov <gladkov.alexey@gmail.com> writes:

We might want to use refcount_t instead of atomic_t. Not a big deal
either way.

> Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> ---
> include/linux/user_namespace.h | 2 +-
> kernel/ucount.c | 10 +++++-----
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 64cf8ebdc4ec..84fefa9247c4 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -92,7 +92,7 @@ struct ucounts {
> struct hlist_node node;
> struct user_namespace *ns;
> kuid_t uid;
> - int count;
> + atomic_t count;
> atomic_t ucount[UCOUNT_COUNTS];
> };
>
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 11b1596e2542..0f2c7c11df19 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -141,7 +141,8 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
>
> new->ns = ns;
> new->uid = uid;
> - new->count = 0;
> +
> + atomic_set(&new->count, 0);
>
> spin_lock_irq(&ucounts_lock);
> ucounts = find_ucounts(ns, uid, hashent);
> @@ -152,10 +153,10 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
> ucounts = new;
> }
> }
> - if (ucounts->count == INT_MAX)
> + if (atomic_read(&ucounts->count) == INT_MAX)
> ucounts = NULL;
> else
> - ucounts->count += 1;
> + atomic_inc(&ucounts->count);
> spin_unlock_irq(&ucounts_lock);
> return ucounts;
> }
> @@ -165,8 +166,7 @@ static void put_ucounts(struct ucounts *ucounts)
> unsigned long flags;
>
> spin_lock_irqsave(&ucounts_lock, flags);
> - ucounts->count -= 1;
> - if (!ucounts->count)
> + if (atomic_dec_and_test(&ucounts->count))
> hlist_del_init(&ucounts->node);
> else
> ucounts = NULL;

This can become:
static void put_ucounts(struct ucounts *ucounts)
{
unsigned long flags;

if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
hlist_del_init(&ucounts->node);
spin_unlock_irqrestore(&ucounts_lock);
kfree(ucounts);