linux-kernel - Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery

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Date:   Sat, 8 Oct 2016 13:58:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Waiman Long <waiman.long@....com>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Rob Clark <robdclark@...il.com>
Subject: Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery

On Fri, 7 Oct 2016, Peter Zijlstra wrote:
> On Fri, Oct 07, 2016 at 04:52:44PM +0200, Peter Zijlstra wrote:
> > Poking at lock internals is not cool. Since I'm going to change the
> > implementation this will break, take it out.
> 
> 
> So something like the below would serve as a replacement for your
> previous hacks. Is this API something acceptable to people? Ingo,
> Thomas?
> 
> ---
>  include/linux/mutex.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> index 4d3bccabbea5..afcff2c85957 100644
> --- a/include/linux/mutex.h
> +++ b/include/linux/mutex.h
> @@ -189,4 +189,29 @@ extern void mutex_unlock(struct mutex *lock);
>  
>  extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
>  
> +enum mutex_trylock_recursive_enum {
> +	mutex_trylock_failed = 0,
> +	mutex_trylock_success = 1,
> +	mutex_trylock_recursive,

Upper case enum symbols please, if at all.

> +};
> +
> +/**
> + * mutex_trylock_recursive - trylock variant that allows recursive locking
> + * @lock: mutex to be locked
> + *
> + *
> + * Returns:
> + *  mutex_trylock_failed    - trylock failed,
> + *  mutex_trylock_success   - lock acquired,
> + *  mutex_trylock_recursive - we already owned the lock.
> + */
> +static inline enum mutex_trylock_recursive_enum
> +mutex_trylock_recursive(struct mutex *lock)
> +{
> +	if (unlikely(__mutex_owner(lock) == current))
> +		return mutex_trylock_recursive;
> +
> +	return mutex_trylock(lock);
> +}

Hmm. I'm not a great fan of this, because that requires an conditional
unlock mechanism.

       res = trylock_recursive(lock);
       if (res == FAILED)
       	       goto out;
       .....

       if (res == SUCCESS)
       	       unlock(lock);

While if you actually keep track of recursion you can do:
  
      if (!trylock_recursive(lock))
		goto out;

      ....

      unlock_recursive(lock);

or even:

     lock_recursive(lock);

     unlock_recursive(lock);

That's making lock/trylock and unlock symetric, so its obvious in the
source what's going on and the recursion tracking allows for better
debugability.

Thanks,

	tglx