linux-kernel - Re: [PATCH] lock/lockdep: Add missing graph_unlock in validate

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Date: Thu, 4 Jan 2024 11:41:47 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Xuewen Yan <xuewen.yan@...soc.com>
Cc: peterz@...radead.org, mingo@...hat.com, will@...nel.org,
	longman@...hat.com, ke.wang@...soc.com, zhiguo.niu@...soc.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lock/lockdep: Add missing graph_unlock in validate_chain

Hi,

On Thu, Jan 04, 2024 at 01:40:30PM +0800, Xuewen Yan wrote:
> The lookup_chain_cache_add will get graph_lock, but the
> validate_chain do not unlock before return 0.
> 

Thanks for looking into this, a few comment below:

> So add graph_unlock before return 0.
> 
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
> ---
>  kernel/locking/lockdep.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 151bd3de5936..24995e1ebc62 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3855,8 +3855,11 @@ static int validate_chain(struct task_struct *curr,
>  		 */
>  		int ret = check_deadlock(curr, hlock);
>  
> -		if (!ret)
> +		if (!ret) {
> +			graph_unlock();

Note that when check_deadlock() return 0, there is a
print_deadlock_bug() before the return, so I think it covers the
graph_unlock() (see debug_locks_off_graph_unlock()).

>  			return 0;
> +		}
> +
>  		/*
>  		 * Add dependency only if this lock is not the head
>  		 * of the chain, and if the new lock introduces no more
> @@ -3865,9 +3868,9 @@ static int validate_chain(struct task_struct *curr,
>  		 * serializes nesting locks), see the comments for
>  		 * check_deadlock().
>  		 */
> -		if (!chain_head && ret != 2) {
> -			if (!check_prevs_add(curr, hlock))
> -				return 0;
> +		if (!chain_head && ret != 2 && !check_prevs_add(curr, hlock)) {
> +			graph_unlock();

This part is interesting, usually when an internal function in lockdep
returns 0, it means there is an error (either a deadlock or internal
error), and that means a print_*() would be called, and the graph lock
will be unlocked in that print_*(). However, in check_prevs_add() there
is one condition where it will return 0 without any print_*(), that is:

in check_prev_add():

			/* <prev> is not found in <next>::locks_before */
			return 0;

it's an internal error where <next> is in the <prev>::locks_after list
but <prev> is not in the <next>::locks_before list, which should seldom
happen: it's dead code. If you put a graph_unlock() before that return,
I think it covers all the cases, unless I'm missing something subtle.

Are you hitting a real issue or this is found by code reading?

Regards,
Boqun

> +			return 0;
>  		}
>  
>  		graph_unlock();
> -- 
> 2.25.1
>