Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage_map iterator

From: Yonghong Song <yhs@fb.com>
To: Jakub Kicinski <kuba@kernel.org>
Cc: <bpf@vger.kernel.org>, <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>, <kernel-team@fb.com>,
	Martin KaFai Lau <kafai@fb.com>
Subject: Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage_map iterator
Date: Tue, 15 Sep 2020 10:35:50 -0700	[thread overview]
Message-ID: <bc24e2da-33e5-e13c-8fe0-1e24c2a5a579@fb.com> (raw)
In-Reply-To: <20200915083327.7e98cf2d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>

On 9/15/20 8:33 AM, Jakub Kicinski wrote:
> On Mon, 14 Sep 2020 11:46:30 -0700 Yonghong Song wrote:
>> Currently, we use bucket_lock when traversing bpf_sk_storage_map
>> elements. Since bpf_iter programs cannot use bpf_sk_storage_get()
>> and bpf_sk_storage_delete() helpers which may also grab bucket lock,
>> we do not have a deadlock issue which exists for hashmap when
>> using bucket_lock ([1]).
>>
>> If a bucket contains a lot of sockets, during bpf_iter traversing
>> a bucket, concurrent bpf_sk_storage_{get,delete}() may experience
>> some undesirable delays. Using rcu_read_lock() is a reasonable
>> compromise here. Although it may lose some precision, e.g.,
>> access stale sockets, but it will not hurt performance of other
>> bpf programs.
>>
>> [1] https://lore.kernel.org/bpf/20200902235341.2001534-1-yhs@fb.com
>>
>> Cc: Martin KaFai Lau <kafai@fb.com>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
> 
> Sparse is not happy about it. Could you add some annotations, perhaps?
> 
> include/linux/rcupdate.h:686:9: warning: context imbalance in 'bpf_sk_storage_map_seq_find_next' - unexpected unlock
> include/linux/rcupdate.h:686:9: warning: context imbalance in 'bpf_sk_storage_map_seq_stop' - unexpected unlock

Okay, I will try.

On my system, sparse is unhappy and core dumped....

/data/users/yhs/work/net-next/include/linux/string.h:12:38: error: too 
many errors
/bin/sh: line 1: 2710132 Segmentation fault      (core dumped) sparse 
-D__linux__ -Dlinux -D__STDC__ -Dunix
-D__unix__ -Wbitwise -Wno-return-void -Wno-unknown-attribute 
-D__x86_64__ --arch=x86 -mlittle-endian -m64 -W
p,-MMD,net/core/.bpf_sk_storage.o.d -nostdinc -isystem
...
/data/users/yhs/work/net-next/net/core/bpf_sk_storage.c
make[3]: *** [net/core/bpf_sk_storage.o] Error 139
make[3]: *** Deleting file `net/core/bpf_sk_storage.o'

-bash-4.4$ rpm -qf /bin/sparse
sparse-0.5.2-1.el7.x86_64
-bash-4.4$