Re: [PATCH] nfsd: fix race between cache_clean and cache_purge

Yihao Wu
2020-03-25T06:37:20Z

On 2020/3/25 1:46 AM, Chuck Lever wrote:
>>>> ---
>>>> net/sunrpc/cache.c | 3 +++
>>>> 1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
>>>> index bd843a81afa0..3e523eefc47f 100644
>>>> --- a/net/sunrpc/cache.c
>>>> +++ b/net/sunrpc/cache.c
>>>> @@ -524,9 +524,11 @@ void cache_purge(struct cache_detail *detail)
>>>> struct hlist_node *tmp = NULL;
>>>> int i = 0;
>>>>
>>>> + spin_lock(&cache_list_lock);
>>>> spin_lock(&detail->hash_lock);
>>>> if (!detail->entries) {
>>>> spin_unlock(&detail->hash_lock);
>>>> + spin_unlock(&cache_list_lock);
>>>> return;
>>>> }
>>>>
>>>> @@ -541,6 +543,7 @@ void cache_purge(struct cache_detail *detail)
>>>> }
>>>> }
>>>> spin_unlock(&detail->hash_lock);
>>>> + spin_unlock(&cache_list_lock);
>>>> }
>>>> EXPORT_SYMBOL_GPL(cache_purge);
>>
>> Hmm... Shouldn't this patch be dropping cache_list_lock() when we call
>> sunrpc_end_cache_remove_entry()? The latter does call both
>> cache_revisit_request() and cache_put(), and while they do not
>> explicitly call anything that holds cache_list_lock, some of those cd-
>>> cache_put callbacks do look as if there is potential for deadlock.
> I see svc_export_put calling dput, eventually, which might_sleep().

Wow that's a little strange. If svc_export_put->dput might_sleep, why can we
spin_lock(&detail->hash_lock); in cache_purge in the first place?

And I agree with Trond those cd->cache_put callbacks are dangerous. I will look
into them today.

But if we dropping cache_list_lock when we call sunrpc_end_cache_remove_entry,
cache_put is not protected, and this patch won't work anymore, right?

Thanks,
Yihao Wu