Hi,
On 12/8/2022 6:28 AM, Paul E. McKenney wrote:
> On Wed, Dec 07, 2022 at 10:24:55AM +0800, Hou Tao wrote:
>> Forget to cc Paul and RCU maillist for more comments.
>>
>> On 12/6/2022 12:29 PM, Hou Tao wrote:
>>> From: Hou Tao <houtao1@xxxxxxxxxx>
>>>
>>> If there are pending rcu callback, free_mem_alloc() will use
>>> rcu_barrier_tasks_trace() and rcu_barrier() to wait for the pending
>>> __free_rcu_tasks_trace() and __free_rcu() callback.
>>>
>>> If rcu_trace_implies_rcu_gp() is true, there will be no pending
>>> __free_rcu(), so it will be OK to skip rcu_barrier() as well.
> The bit about there being no pending __free_rcu() is guaranteed by
> your algorithm, correct? As in you have something like this somewhere
> else in the code?
>
> if (!rcu_trace_implies_rcu_gp())
> call_rcu(...);
>
> Or am I missing something more subtle?
Yes. It is guaranteed by the implementation of bpf mem allocator: if
rcu_trace_implies_rcu_gp() is true, there will be no call_rcu() in bpf memory
allocator.
>
> Thanx, Paul
>
>>> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
>>> ---
>>> kernel/bpf/memalloc.c | 10 +++++++++-
>>> 1 file changed, 9 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
>>> index 7daf147bc8f6..d43991fafc4f 100644
>>> --- a/kernel/bpf/memalloc.c
>>> +++ b/kernel/bpf/memalloc.c
>>> @@ -464,9 +464,17 @@ static void free_mem_alloc(struct bpf_mem_alloc *ma)
>>> {
>>> /* waiting_for_gp lists was drained, but __free_rcu might
>>> * still execute. Wait for it now before we freeing percpu caches.
>>> + *
>>> + * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
>>> + * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
>>> + * to wait for the pending __free_rcu_tasks_trace() and __free_rcu(),
>>> + * so if call_rcu(head, __free_rcu) is skipped due to
>>> + * rcu_trace_implies_rcu_gp(), it will be OK to skip rcu_barrier() by
>>> + * using rcu_trace_implies_rcu_gp() as well.
>>> */
>>> rcu_barrier_tasks_trace();
>>> - rcu_barrier();
>>> + if (!rcu_trace_implies_rcu_gp())
>>> + rcu_barrier();
>>> free_mem_alloc_no_barrier(ma);
>>> }
>>>
