Message-ID: <55ca60f3-3841-4f6a-b757-04d52ca1f1a1@linux.dev>
Date: Tue, 5 Aug 2025 16:10:19 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Amery Hung <ameryhung@...il.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
 alexei.starovoitov@...il.com, andrii@...nel.org, daniel@...earbox.net,
 memxor@...il.com, kpsingh@...nel.org, martin.lau@...nel.org,
 yonghong.song@...ux.dev, song@...nel.org, haoluo@...gle.com,
 kernel-team@...a.com
Subject: Re: [RFC PATCH bpf-next v1 03/11] bpf: Open code
 bpf_selem_unlink_storage in bpf_selem_unlink

On 8/5/25 9:25 AM, Amery Hung wrote:
> On Fri, Aug 1, 2025 at 5:58 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>>
>> On 7/29/25 11:25 AM, Amery Hung wrote:
>>>    void bpf_selem_unlink(struct bpf_local_storage_elem *selem, bool reuse_now)
>>>    {
>>> +     struct bpf_local_storage_map *storage_smap;
>>> +     struct bpf_local_storage *local_storage = NULL;
>>> +     bool bpf_ma, free_local_storage = false;
>>> +     HLIST_HEAD(selem_free_list);
>>>        struct bpf_local_storage_map_bucket *b;
>>> -     struct bpf_local_storage_map *smap;
>>> -     unsigned long flags;
>>> +     struct bpf_local_storage_map *smap = NULL;
>>> +     unsigned long flags, b_flags;
>>>
>>>        if (likely(selem_linked_to_map_lockless(selem))) {
>>
>> Can we simplify the bpf_selem_unlink() function by skipping this map_lockless
>> check,
>>
>>>                smap = rcu_dereference_check(SDATA(selem)->smap, bpf_rcu_lock_held());
>>>                b = select_bucket(smap, selem);
>>> -             raw_spin_lock_irqsave(&b->lock, flags);
>>> +     }
>>>
>>> -             /* Always unlink from map before unlinking from local_storage
>>> -              * because selem will be freed after successfully unlinked from
>>> -              * the local_storage.
>>> -              */
>>> -             bpf_selem_unlink_map_nolock(selem);
>>> -             raw_spin_unlock_irqrestore(&b->lock, flags);
>>> +     if (likely(selem_linked_to_storage_lockless(selem))) {
>>
>> only depends on this and then proceed to take the lock_storage->lock. Then
>> recheck selem_linked_to_storage(selem), bpf_selem_unlink_map(selem) first, and
>> then bpf_selem_unlink_storage_nolock(selem) last.
> 
> Thanks for the suggestion. I think it will simplify the function. Just
> making sure I am getting you right, you mean instead of open code both
> unlink_map and unlink_storage, only open code unlink_storage. First,
> grab local_storage->lock and call bpf_selem_unlink_map(). Then, only

After grabbing the local-storage->lock, re-check selem_linked_to_storage() first 
before calling bpf_selem_unlink_map().

> proceed to unlink_storage only If bpf_selem_unlink_map() succeeds.

No strong opinion on open coding bpf_selem_unlink_map() or not. I think they are 
the same. I reuse the bpf_selem_unlink_map() because I think it can be used as 
is. The logic of bpf_selem_unlink() here should be very similar to the 
bpf_local_storage_destroy() now except it needs to recheck the 
selem_linked_to_storage():

1. grab both locks.
2. If selem_linked_to_storage() is true, the selem_linked_to_map() should also 
be true since we now need to grab both locks before moving forward to unlink. 
Meaning either a selem will not be unlinked at all or it will be unlinked from 
both local_storage and map. Am I thinking it correctly or there is hole?

> 
>>
>> Then bpf_selem_unlink_map can use selem->local_storage->owner to select_bucket().
> 
> Not sure what this part mean. Could you elaborate?

I meant to pass owner pointer to select_bucket, like
select_bucket(smap, selem->local_storage->owner). Of course, the owner pointer 
should also be used on the update side also, i.e. bpf_local_storage_update().
Then it does not need to take the second bucket lock when "if (b != old_b)" in 
the bpf_local_storage_update() in patch 1.

> 
>>
>>> +             local_storage = rcu_dereference_check(selem->local_storage,
>>> +                                                   bpf_rcu_lock_held());
>>> +             storage_smap = rcu_dereference_check(local_storage->smap,
>>> +                                                  bpf_rcu_lock_held());
>>> +             bpf_ma = check_storage_bpf_ma(local_storage, storage_smap, selem);
>>>        }
>>>
>>> -     bpf_selem_unlink_storage(selem, reuse_now);
>>> +     if (local_storage)
>>> +             raw_spin_lock_irqsave(&local_storage->lock, flags);
>>> +     if (smap)
>>> +             raw_spin_lock_irqsave(&b->lock, b_flags);
>>> +
>>> +     /* Always unlink from map before unlinking from local_storage
>>> +      * because selem will be freed after successfully unlinked from
>>> +      * the local_storage.
>>> +      */
>>> +     if (smap)

This "if (smap)" test

>>> +             bpf_selem_unlink_map_nolock(selem);
>>> +     if (local_storage && likely(selem_linked_to_storage(selem)))

and this orthogonal "if (local_storage && 
likely(selem_linked_to_storage(selem)))" test.

Understood it is from the existing codes that unlink from map and unlink from 
local_storage can be done independently. It was there because it did not need to 
grab both locks to move forward. Since now it needs both locks, I think we can 
simplify things a bit like mentioned above.


>>> +             free_local_storage = bpf_selem_unlink_storage_nolock(
>>> +                     local_storage, selem, true, &selem_free_list);
>>> +
>>> +     if (smap)
>>> +             raw_spin_unlock_irqrestore(&b->lock, b_flags);
>>> +     if (local_storage)
>>> +             raw_spin_unlock_irqrestore(&local_storage->lock, flags);
>>> +
>>> +     bpf_selem_free_list(&selem_free_list, reuse_now);
>>> +
>>> +     if (free_local_storage)
>>> +             bpf_local_storage_free(local_storage, storage_smap, bpf_ma, reuse_now);
>>>    }
>>>
>>>    void __bpf_local_storage_insert_cache(struct bpf_local_storage *local_storage,
>>

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