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

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Message-ID: <CAB8ipk_hcbYbZhoV0ZCDH0smQPTz98CQH7wP=PjJS6kcxavpVg@mail.gmail.com>
Date: Tue, 9 Jan 2024 13:11:23 +0800
From: Xuewen Yan <xuewen.yan94@...il.com>
To: Waiman Long <longman@...hat.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

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;
 }


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
>
>