linux-kernel - Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage

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Date:   Tue, 23 Feb 2021 20:49:32 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive
 bpf_task_storage_[get|delete]



> On Feb 23, 2021, at 3:06 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, Feb 22, 2021 at 05:20:10PM -0800, Song Liu wrote:
>> BPF helpers bpf_task_storage_[get|delete] could hold two locks:
>> bpf_local_storage_map_bucket->lock and bpf_local_storage->lock. Calling
>> these helpers from fentry/fexit programs on functions in bpf_*_storage.c
>> may cause deadlock on either locks.
>> 
>> Prevent such deadlock with a per cpu counter, bpf_task_storage_busy, which
>> is similar to bpf_prog_active. We need this counter to be global, because
> 
> So bpf_prog_active is one of the biggest turds around, and now you're
> making it worse ?!

bpf_prog_active is a distraction here. We are trying to enable task local 
storage for fentry/fext programs, which do not use bpf_prog_active.

bpf_task_storage_busy counter is introduced to protect against a specific 
pattern of deadlocks (attaching fentry/fexit on bpf_task_storage_[get|delete] 
helpers, then let the programs call these two helpers again). 

Thanks,
Song