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Date:   Thu, 27 Aug 2020 21:01:01 -0400
From:   Josef Bacik <josef@...icpanda.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        davem@...emloft.net
Cc:     daniel@...earbox.net, bpoirier@...e.com, akpm@...ux-foundation.org,
        hannes@...xchg.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v3 bpf-next 2/5] bpf: Introduce sleepable BPF programs

On 8/27/20 6:01 PM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> Introduce sleepable BPF programs that can request such property for themselves
> via BPF_F_SLEEPABLE flag at program load time. In such case they will be able
> to use helpers like bpf_copy_from_user() that might sleep. At present only
> fentry/fexit/fmod_ret and lsm programs can request to be sleepable and only
> when they are attached to kernel functions that are known to allow sleeping.
> 
> The non-sleepable programs are relying on implicit rcu_read_lock() and
> migrate_disable() to protect life time of programs, maps that they use and
> per-cpu kernel structures used to pass info between bpf programs and the
> kernel. The sleepable programs cannot be enclosed into rcu_read_lock().
> migrate_disable() maps to preempt_disable() in non-RT kernels, so the progs
> should not be enclosed in migrate_disable() as well. Therefore
> rcu_read_lock_trace is used to protect the life time of sleepable progs.
> 
> There are many networking and tracing program types. In many cases the
> 'struct bpf_prog *' pointer itself is rcu protected within some other kernel
> data structure and the kernel code is using rcu_dereference() to load that
> program pointer and call BPF_PROG_RUN() on it. All these cases are not touched.
> Instead sleepable bpf programs are allowed with bpf trampoline only. The
> program pointers are hard-coded into generated assembly of bpf trampoline and
> synchronize_rcu_tasks_trace() is used to protect the life time of the program.
> The same trampoline can hold both sleepable and non-sleepable progs.
> 
> When rcu_read_lock_trace is held it means that some sleepable bpf program is
> running from bpf trampoline. Those programs can use bpf arrays and preallocated
> hash/lru maps. These map types are waiting on programs to complete via
> synchronize_rcu_tasks_trace();
> 
> Updates to trampoline now has to do synchronize_rcu_tasks_trace() and
> synchronize_rcu_tasks() to wait for sleepable progs to finish and for
> trampoline assembly to finish.
> 
> This is the first step of introducing sleepable progs. Eventually dynamically
> allocated hash maps can be allowed and networking program types can become
> sleepable too.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> Acked-by: Andrii Nakryiko <andriin@...com>

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef

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