[v2] lockdep: Add missing graph_unlock in check_prev_add

  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@unisoc.com>
Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.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(+)
  

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@unisoc.com>
> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.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,

It looks like this bug was first introduced by commit 910b1b2e6d 
("[PATCH] lockdep: internal locking fixes"). So you may also add a fixes 
tag.

Cheers,
Longman
  

Hi Waiman


On Tue, Jan 9, 2024 at 11:51 AM Waiman Long <longman@redhat.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@unisoc.com>
> > Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.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;
 }


Thanks!
---
BRs
xuewen

> It looks like this bug was first introduced by commit 910b1b2e6d
> ("[PATCH] lockdep: internal locking fixes"). So you may also add a fixes
> tag.
>
> Cheers,
> Longman
>
>
  

On 1/9/24 00:11, Xuewen Yan wrote:
> Hi Waiman
>
>
> On Tue, Jan 9, 2024 at 11:51 AM Waiman Long <longman@redhat.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@unisoc.com>
>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.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