Hi,
On 6/28/2023 8:56 AM, Alexei Starovoitov wrote:
> 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@xxxxxxxxxx>
>>>
>>> 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.
For v2, it was also hard for me to reproduce the leak problem. But after
I injected some delay by using udelay() in __free_by_rcu/__free_rcu()
after reading c->draining, it was relatively easy to reproduce the problems.
