linux-kernel - Re: [PATCH v2] lockdep: Add missing graph_unlock in check_prev

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Date: Tue, 9 Jan 2024 10:40:03 -0500
From: Waiman Long <longman@...hat.com>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, peterz@...radead.org,
 mingo@...hat.com, will@...nel.org, boqun.feng@...il.com,
 zhiguo.niu@...soc.com, ke.wang@...soc.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] lockdep: Add missing graph_unlock in check_prev_add

On 1/9/24 00:11, Xuewen Yan wrote:
> Hi Waiman
>
>
> On Tue, Jan 9, 2024 at 11:51 AM Waiman Long <longman@...hat.com> wrote:
>> On 1/5/24 01:04, Xuewen Yan wrote:
>>> The check_prev_add() is held graph_lock, and it should unlock
>>> the graph_lock before return 0.
>>> But there is one condition where it will return 0 without unlock,
>>> that is:
>>>
>>> /* <prev> is not found in <next>::locks_before */
>>>        return 0;
>>>
>>> So add graph_unlock before return 0.
>>>
>>> Fixes: 3454a36d6a39 ("lockdep: Introduce lock_list::dep")
>>> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
>>> ---
>>> Change in V2:
>>> -move the graph_unlock to check_prev_add from validate_chain(Boqun)
>>> -Add fix tag
>>> ---
>>> ---
>>>    kernel/locking/lockdep.c | 1 +
>>>    1 file changed, 1 insertion(+)
>>>
>>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>>> index 151bd3de5936..c8602a251bec 100644
>>> --- a/kernel/locking/lockdep.c
>>> +++ b/kernel/locking/lockdep.c
>>> @@ -3178,6 +3178,7 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
>>>                        }
>>>
>>>                        /* <prev> is not found in <next>::locks_before */
>>> +                     graph_unlock();
>>>                        return 0;
>>>                }
>>>        }
>> There are multiple places in check_prev_add() that will return 0. It
>> will be odd to have just one of them has a graph_unlock(). It makes the
>> code hard to understand. You should insert graph_unlock() in a place
>> that matches the other places where graph_unlock() will be called. My
>> suggestion is as follows:
>>
>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index 151bd3de5936..d9f2df36332c 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -3252,7 +3252,7 @@ check_prevs_add(struct task_struct *curr, struct
>> held_loc>
>>                   if (hlock->check) {
>>                           int ret = check_prev_add(curr, hlock, next,
>> distance, &>
>>                           if (!ret)
>> -                               return 0;
>> +                               goto out_bug;
>>
>>                           /*
>>                            * Stop after the first non-trylock entry,
>>
> As you say, there are multiple places in check_prev_add() that will
> return 0, and some cases had unlocked the lock, if all goto the
> out_bug, would it cause double unlock?
> Maybe as follows?
> ---
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 151bd3de5936..8b665ba90ad0 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3178,7 +3178,7 @@ check_prev_add(struct task_struct *curr, struct
> held_lock *prev,
>                          }
>
>                          /* <prev> is not found in <next>::locks_before */
> -                       return 0;
> +                       goto list_err;
>                  }
>          }
>
> @@ -3215,6 +3215,11 @@ check_prev_add(struct task_struct *curr, struct
> held_lock *prev,
>                  return 0;
>
>          return 2;
> +
> +list_err:
> +       /* still get graph_lock, unlock it before return*/
> +       graph_unlock();
> +       return 0;
>   }

I see. the graph_unlock() is called before any error message is printed. 
I understand the reason why this is done this way, but it does make it 
easy to re-introduce this kind of error when the lockdep code is 
changed. We need a better system to track the state of the graph_lock 
and do an unlock if necessary.

Cheers,
Longman