netdev - Re: [PATCH v6 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage

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Date:   Thu, 25 Feb 2021 16:01:57 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Song Liu <songliubraving@...com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <ast@...nel.org>,
        <daniel@...earbox.net>, <kernel-team@...com>,
        <peterz@...radead.org>
Subject: Re: [PATCH v6 bpf-next 2/6] bpf: prevent deadlock from recursive
 bpf_task_storage_[get|delete]

On Thu, Feb 25, 2021 at 03:43:15PM -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. We
> need this counter to be global, because the two locks here belong to two
> different objects: bpf_local_storage_map and bpf_local_storage. If we
> pick one of them as the owner of the counter, it is still possible to
> trigger deadlock on the other lock. For example, if bpf_local_storage_map
> owns the counters, it cannot prevent deadlock on bpf_local_storage->lock
> when two maps are used.
Acked-by: Martin KaFai Lau <kafai@...com>