Hi,
On 7/7/2023 10:12 AM, Alexei Starovoitov wrote:
> On Thu, Jul 6, 2023 at 7:07 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
>> Hi,
>>
>> On 7/6/2023 11:34 AM, Alexei Starovoitov wrote:
>>> From: Alexei Starovoitov <ast@xxxxxxxxxx>
>>>
>>> alloc_bulk() can reuse elements from free_by_rcu_ttrace.
>>> Let it reuse from waiting_for_gp_ttrace as well to avoid unnecessary kmalloc().
>>>
>>> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
>>> ---
>>> kernel/bpf/memalloc.c | 16 ++++++++++------
>>> 1 file changed, 10 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
>>> index 9986c6b7df4d..e5a87f6cf2cc 100644
>>> --- a/kernel/bpf/memalloc.c
>>> +++ b/kernel/bpf/memalloc.c
>>> @@ -212,6 +212,15 @@ static void alloc_bulk(struct bpf_mem_cache *c, int cnt, int node)
>>> if (i >= cnt)
>>> return;
>>>
>>> + for (; i < cnt; i++) {
>>> + obj = llist_del_first(&c->waiting_for_gp_ttrace);
>>> + if (!obj)
>>> + break;
>>> + add_obj_to_free_list(c, obj);
>>> + }
>>> + if (i >= cnt)
>>> + return;
>> I still think using llist_del_first() here is not safe as reported in
>> [1]. Not sure whether or not invoking enque_to_free() firstly for
>> free_llist_extra will close the race completely. Will check later.
> lol. see my reply a second ago in the other thread.
>
> and it's not just waiting_for_gp_ttrace. free_by_rcu_ttrace is similar.
I think free_by_rcu_ttrace is different, because the reuse is only
possible after one tasks trace RCU grace period as shown below, and the
concurrent llist_del_first() must have been completed when the head is
reused and re-added into free_by_rcu_ttrace again.
// c0->free_by_rcu_ttrace
A -> B -> C -> nil
P1:
alloc_bulk()
llist_del_first(&c->free_by_rcu_ttrace)
entry = A
next = B
P2:
do_call_rcu_ttrace()
// c->free_by_rcu_ttrace->first = NULL
llist_del_all(&c->free_by_rcu_ttrace)
move to c->waiting_for_gp_ttrace
P1:
llist_del_first()
return NULL
// A is only reusable after one task trace RCU grace
// llist_del_first() must have been completed
__free_rcu_tasks_trace
free_all(llist_del_all(&c->waiting_for_gp_ttrace))
