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Date:   Mon, 9 Jan 2023 17:52:37 -0800
MIME-Version: 1.0
Subject: Re: [bpf-next v4 1/2] bpf: hash map, avoid deadlock with suitable
 hash mask
Content-Language: en-US
To:     tong@infragraf.org, bpf@vger.kernel.org
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Hou Tao <houtao1@huawei.com>
References: <20230105092637.35069-1-tong@infragraf.org>
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From:   Martin KaFai Lau <martin.lau@linux.dev>
In-Reply-To: <20230105092637.35069-1-tong@infragraf.org>
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On 1/5/23 1:26 AM, tong@infragraf.org wrote:
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 5aa2b5525f79..974f104f47a0 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_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);
>   
>   	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_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);

Please run checkpatch.pl.  patchwork also reports the same thing:
https://patchwork.kernel.org/project/netdevbpf/patch/20230105092637.35069-1-tong@infragraf.org/

CHECK: spaces preferred around that '-' (ctx:WxV)
#46: FILE: kernel/bpf/hashtab.c:155:
+	hash = hash & min_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);
  	                                                                ^

CHECK: spaces preferred around that '-' (ctx:WxV)
#55: FILE: kernel/bpf/hashtab.c:174:
+	hash = hash & min_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets -1);

btw, instead of doing this min_t and -1 repeatedly, ensuring n_buckets is at 
least HASHTAB_MAP_LOCK_COUNT during map_alloc should be as good?  htab having 2 
or 4 max_entries should be pretty uncommon.