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Subject: Re: [PATCH bpf] Use call_rcu_hurry() with synchronize_rcu_mult()
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Cc: Alexei Starovoitov <ast@kernel.org>,
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References: <358bde93-4933-4305-ac42-4d6f10c97c08@paulmck-laptop>
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	lindbergh.monkeyblade.net

On 5/18/23 7:47 AM, Paul E. McKenney wrote:
> The bpf_struct_ops_map_free() function must wait for both an RCU grace
> period and an RCU Tasks grace period, and so it passes call_rcu() and
> call_rcu_tasks() to synchronize_rcu_mult().  This works, but on ChromeOS
> and Android platforms call_rcu() can have lazy semantics, resulting in
> multi-second delays between call_rcu() invocation and invocation of the
> corresponding callback.
> 
> Therefore, substitute call_rcu_hurry() for call_rcu().

My understanding on the net-effect is to free up the struct_ops resources faster.

I believe call_rcu() should be fine. struct_ops freeing should not happen very 
often. For example, when a bpf written tcp congestion control (struct_ops) is 
registered, it will stay in the kernel for a long time. A couple seconds delay 
in releasing the struct_ops should be acceptable.