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Date:   Mon, 17 Oct 2022 15:26:41 -0700
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Subject: Re: [PATCH bpf-next 2/5] bpf: Implement cgroup storage available to
 non-cgroup-attached bpf progs
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To:     sdf@google.com
Cc:     bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, kernel-team@fb.com,
        KP Singh <kpsingh@kernel.org>,
        Martin KaFai Lau <martin.lau@kernel.org>,
        Tejun Heo <tj@kernel.org>, Yonghong Song <yhs@fb.com>
References: <20221014045619.3309899-1-yhs@fb.com>
 <20221014045630.3311951-1-yhs@fb.com> <Y02Yk8gUgVDuZR4Q@google.com>
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From:   Martin KaFai Lau <martin.lau@linux.dev>
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On 10/17/22 11:01 AM, sdf@google.com wrote:
>> +static bool bpf_cgroup_storage_trylock(void)
>> +{
>> +    migrate_disable();
>> +    if (unlikely(this_cpu_inc_return(bpf_cgroup_storage_busy) != 1)) {
>> +        this_cpu_dec(bpf_cgroup_storage_busy);
>> +        migrate_enable();
>> +        return false;
>> +    }
>> +    return true;
>> +}
> 
> Task storage has lock/unlock/trylock; inode storage doesn't; why does
> cgroup need it as well?

This was added in bc235cdb423a2 to avoid deadlock for tracing program which can 
get a hold to the same task ptr easily with bpf_get_current_task_btf().  I 
believe there was no known way to hit this problem in inode storage, so inode 
storage does not use it.

The common tracing use case to get a hold of the cgroup ptr is through task 
(including bpf_get_current_task_btf()), so it seems to make sense to mimic the 
trylock here.  I have plan to relax it for all non-tracing programs like 
cgroup-bpf and bpf-lsm.