netdev - Re: [PATCH bpf-next] bpf: using rcu_read_lock for bpf_sk_storage

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Message-ID: <20200915104001.0182ae73@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 15 Sep 2020 10:40:01 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yonghong Song <yhs@...com>
Cc:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next] bpf: using rcu_read_lock for
 bpf_sk_storage_map iterator

On Tue, 15 Sep 2020 10:35:50 -0700 Yonghong Song wrote:
> 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@...com>
> >> Signed-off-by: Yonghong Song <yhs@...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$

I think you need to build from source, sadly :(

https://git.kernel.org/pub/scm//devel/sparse/sparse.git