netdev - Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Tue, 27 Jun 2023 17:56:10 -0700
From: Alexei Starovoitov <ast@...a.com>
To: Hou Tao <houtao@...weicloud.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        daniel@...earbox.net, andrii@...nel.org, void@...ifault.com,
        paulmck@...nel.org
Cc: tj@...nel.org, rcu@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

On 6/25/23 4:15 AM, Hou Tao wrote:
> Hi,
> 
> On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
>> From: Alexei Starovoitov <ast@...nel.org>
>>
>> Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
>> Unlike bpf_mem_[cache_]free() that links objects for immediate reuse into
>> per-cpu free list the _rcu() flavor waits for RCU grace period and then moves
>> objects into free_by_rcu_ttrace list where they are waiting for RCU
>> task trace grace period to be freed into slab.
> SNIP
>>   
>>   static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
>> @@ -498,8 +566,8 @@ static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
>>   
>>   static void free_mem_alloc(struct bpf_mem_alloc *ma)
>>   {
>> -	/* waiting_for_gp_ttrace lists was drained, but __free_rcu might
>> -	 * still execute. Wait for it now before we freeing percpu caches.
>> +	/* waiting_for_gp[_ttrace] lists were drained, but RCU callbacks
>> +	 * might still execute. Wait for them.
>>   	 *
>>   	 * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
>>   	 * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
> I think an extra rcu_barrier() before rcu_barrier_tasks_trace() is still
> needed here, otherwise free_mem_alloc will not wait for inflight
> __free_by_rcu() and there will oops in rcu_do_batch().

Agree. I got confused by rcu_trace_implies_rcu_gp().
rcu_barrier() is necessary.

re: draining.
I'll switch to do if (draing) free_all; else call_rcu; scheme
to address potential memory leak though I wasn't able to repro it.