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

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Date: Mon, 8 Jan 2024 22:49:30 -0500
From: Waiman Long <longman@...hat.com>
To: Xuewen Yan <xuewen.yan@...soc.com>, peterz@...radead.org,
 mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Cc: 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 1/5/24 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,

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