Re: [bpf-next 1/2] bpf: hash map, avoid deadlock with suitable hash mask

[Hou Tao <houtao1@huawei.com>]

Hi,

On 12/14/2022 6:38 PM, xiangxia.m.yue@gmail.com wrote:
> From: Tonghao Zhang <xiangxia.m.yue@gmail.com>
>
> The deadlock still may occur while accessed in NMI and non-NMI
> context. Because in NMI, we still may access the same bucket but with
> different map_locked index.
>
> For example, on the same CPU, .max_entries = 2, we update the hash map,
> with key = 4, while running bpf prog in NMI nmi_handle(), to update
> hash map with key = 20, so it will have the same bucket index but have
> different map_locked index.
>
> To fix this issue, using min mask to hash again.
>
> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Andrii Nakryiko <andrii@kernel.org>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Song Liu <song@kernel.org>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: Stanislav Fomichev <sdf@google.com>
> Cc: Hao Luo <haoluo@google.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Hou Tao <houtao1@huawei.com>
> ---
>  kernel/bpf/hashtab.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 5aa2b5525f79..8b25036a8690 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -152,7 +152,7 @@ static inline int htab_lock_bucket(const struct bpf_htab *htab,
>  {
>  	unsigned long flags;
>  
> -	hash = hash & HASHTAB_MAP_LOCK_MASK;
> +	hash = hash & min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);
There is warning for kernel test robot and it seems that min_t(...) is required
here.

Otherwise, this patch looks good to me, so:

Acked-by: Hou Tao <houtao1@huawei.com>
>  
>  	preempt_disable();
>  	if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
> @@ -171,7 +171,7 @@ static inline void htab_unlock_bucket(const struct bpf_htab *htab,
>  				      struct bucket *b, u32 hash,
>  				      unsigned long flags)
>  {
> -	hash = hash & HASHTAB_MAP_LOCK_MASK;
> +	hash = hash & min(HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);
>  	raw_spin_unlock_irqrestore(&b->raw_lock, flags);
>  	__this_cpu_dec(*(htab->map_locked[hash]));
>  	preempt_enable();