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

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Date:   Sat, 10 Jun 2017 16:02:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
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 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.

> Make rt_mutex_init_proxy_locked pass a name and a key for the lock.
> 
> Fixes: f5694788ad8d ("rt_mutex: Add lockdep annotations")
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
> ---
>  kernel/locking/rtmutex.c        |  6 ++++--
>  kernel/locking/rtmutex_common.h | 12 ++++++++++--
>  2 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 43123533e9b1..f540961cec30 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1679,10 +1679,12 @@ EXPORT_SYMBOL_GPL(__rt_mutex_init);
>   * possible at this point because the pi_state which contains the rtmutex
>   * is not yet visible to other tasks.
>   */
> -void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
> +void __rt_mutex_init_proxy_locked(struct rt_mutex *lock,
> +				const char *name,
> +				struct lock_class_key *key,
>  				struct task_struct *proxy_owner)
>  {
> -	__rt_mutex_init(lock, NULL, NULL);
> +	__rt_mutex_init(lock, name, key);
>  	debug_rt_mutex_proxy_lock(lock, proxy_owner);
>  	rt_mutex_set_owner(lock, proxy_owner);
>  }

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
 }