Re: [PATCH 1/3] bpf: Use spinlock_t in bpf_lru_list

[Yonghong Song <yhs@fb.com>]

On 4/10/19 7:30 AM, Sebastian Andrzej Siewior wrote:
> There is no difference between spinlock_t and raw_spinlock_t for !RT
> kernels. It is possible that bpf_lru_list_pop_free_to_local() invokes
> at least three list walks which look unbounded it probably makes sense
> to use spinlock_t.
> 
> Make bpf_lru_list use a spinlock_t.

Could you add a cover letter for the patch set since you have
more than one patch?

> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>   kernel/bpf/bpf_lru_list.c | 38 +++++++++++++++++++-------------------
>   kernel/bpf/bpf_lru_list.h |  4 ++--
>   2 files changed, 21 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/bpf/bpf_lru_list.c b/kernel/bpf/bpf_lru_list.c
> index e6ef4401a1380..40f47210c3817 100644
> --- a/kernel/bpf/bpf_lru_list.c
> +++ b/kernel/bpf/bpf_lru_list.c
> @@ -313,9 +313,9 @@ static void bpf_lru_list_push_free(struct bpf_lru_list *l,
>   	if (WARN_ON_ONCE(IS_LOCAL_LIST_TYPE(node->type)))
>   		return;
>   
> -	raw_spin_lock_irqsave(&l->lock, flags);
> +	spin_lock_irqsave(&l->lock, flags);
>   	__bpf_lru_node_move(l, node, BPF_LRU_LIST_T_FREE);
> -	raw_spin_unlock_irqrestore(&l->lock, flags);
> +	spin_unlock_irqrestore(&l->lock, flags);
>   }

This function (plus many others) is called by bpf program helpers which 
cannot sleep. Is it possible that under RT spin_lock_irqsave() could 
sleep and this will make bpf subsystem cannot be used under RT?

>   
>   static void bpf_lru_list_pop_free_to_local(struct bpf_lru *lru,
> @@ -325,7 +325,7 @@ static void bpf_lru_list_pop_free_to_local(struct bpf_lru *lru,
>   	struct bpf_lru_node *node, *tmp_node;
>   	unsigned int nfree = 0;
>   
> -	raw_spin_lock(&l->lock);
> +	spin_lock(&l->lock);
>   
>   	__local_list_flush(l, loc_l);
>   
> @@ -344,7 +344,7 @@ static void bpf_lru_list_pop_free_to_local(struct bpf_lru *lru,
>   				      local_free_list(loc_l),
>   				      BPF_LRU_LOCAL_LIST_T_FREE);
>   
> -	raw_spin_unlock(&l->lock);
> +	spin_unlock(&l->lock);
>   }
>   
>   static void __local_list_add_pending(struct bpf_lru *lru,
[...]
>   static void bpf_lru_list_init(struct bpf_lru_list *l)
> @@ -642,7 +642,7 @@ static void bpf_lru_list_init(struct bpf_lru_list *l)
>   
>   	l->next_inactive_rotation = &l->lists[BPF_LRU_LIST_T_INACTIVE];
>   
> -	raw_spin_lock_init(&l->lock);
> +	spin_lock_init(&l->lock);
>   }
>   
>   int bpf_lru_init(struct bpf_lru *lru, bool percpu, u32 hash_offset,
> diff --git a/kernel/bpf/bpf_lru_list.h b/kernel/bpf/bpf_lru_list.h
> index 7d4f89b7cb841..4e1e4608f1bb0 100644
> --- a/kernel/bpf/bpf_lru_list.h
> +++ b/kernel/bpf/bpf_lru_list.h
> @@ -36,13 +36,13 @@ struct bpf_lru_list {
>   	/* The next inacitve list rotation starts from here */
>   	struct list_head *next_inactive_rotation;
>   
> -	raw_spinlock_t lock ____cacheline_aligned_in_smp;
> +	spinlock_t lock ____cacheline_aligned_in_smp;
>   };
>   
>   struct bpf_lru_locallist {
>   	struct list_head lists[NR_BPF_LRU_LOCAL_LIST_T];
>   	u16 next_steal;
> -	raw_spinlock_t lock;
> +	spinlock_t lock;
>   };
>   
>   struct bpf_common_lru {
>