Re: [PATCH v3 3/4] nfsd: close race between unhashing and LRU addition

[Chuck Lever III <chuck.lever@oracle.com>]

> On Oct 31, 2022, at 6:08 AM, Jeff Layton <jlayton@kernel.org> wrote:
> 
> On Mon, 2022-10-31 at 02:51 +0000, Chuck Lever III wrote:
>> 
>>> On Oct 30, 2022, at 5:45 PM, NeilBrown <neilb@suse.de> wrote:
>>> 
>>> On Sat, 29 Oct 2022, Chuck Lever III wrote:
>>>> 
>>>>> On Oct 28, 2022, at 2:57 PM, Jeff Layton <jlayton@kernel.org> wrote:
>>>>> 
>>>>> The list_lru_add and list_lru_del functions use list_empty checks to see
>>>>> whether the object is already on the LRU. That's fine in most cases, but
>>>>> we occasionally repurpose nf_lru after unhashing. It's possible for an
>>>>> LRU removal to remove it from a different list altogether if we lose a
>>>>> race.
>> 
>> Can that issue be resolved by simply adding a "struct list_head nf_dispose"
>> field? That might be more straightforward than adding conditional logic.
>> 
> 
> Yes, though that would take more memory.

Not really. pahole says struct nfsd_file is currently 40 bytes short
of two cache lines. So adding a list_head field should not push the
size of nfsd_file to the point where kmalloc would have to allocate
more memory per object.

I'm wondering if a separate list_head field would help simplify
nfsd_file_put() ?


--
Chuck Lever