linux-kernel - Re: [PATCH] rt_mutex: correctly initialize lockdep in rt_mutex_init_proxy

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Message-ID: <20170611145122.j3oxf6nsswc5okr7@sasha-lappy>
Date:   Sun, 11 Jun 2017 14:51:09 +0000
From:   "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rt_mutex: correctly initialize lockdep in
 rt_mutex_init_proxy_locked

On Sat, Jun 10, 2017 at 04:02:12PM +0200, Peter Zijlstra wrote:
> On Sat, Jun 10, 2017 at 02:48:04AM +0000, Levin, Alexander (Sasha Levin) wrote:
> > lockdep can't deal with NULL name or key, and doesn't do anything
> > with the lock when that happens.
> 
> Not doing anything is 'right', the proxy stuff won't be lockdep tracked
> anyway. But yeah, the first thing is a wee bit of a problem, for it will
> trigger DEBUG_LOCKS_WARN_ON() and fully kill lockdep.

But don't we want pi_state->pi_mutex tracked by lockdep?

> Yeah, no need to do that; all we really need here is something like:
> 
> ---
>  kernel/locking/rtmutex-debug.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/locking/rtmutex-debug.c b/kernel/locking/rtmutex-debug.c
> index ac35e648b0e5..8dc647dc4b4b 100644
> --- a/kernel/locking/rtmutex-debug.c
> +++ b/kernel/locking/rtmutex-debug.c
> @@ -175,7 +175,8 @@ void debug_rt_mutex_init(struct rt_mutex *lock, const char *name, struct lock_cl
>  	lock->name = name;
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> -	lockdep_init_map(&lock->dep_map, name, key, 0);
> +	if (name && key)
> +		lockdep_init_map(&lock->dep_map, name, key, 0);
>  #endif
>  }

I didn't want to do that because in later calls on that mutex we
will end up going into lockdep code, and I didn't think that doing
that without calling lockdep_init_map() initially was safe.

-- 

Thanks,
Sasha