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Date: Sat, 8 Oct 2016 16:01:36 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Waiman Long <waiman.long@hpe.com>, Jason Low <jason.low2@hpe.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
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        Chris Wilson <chris@chris-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Rob Clark <robdclark@gmail.com>
Subject: Re: [PATCH -v4 1/8] locking/drm: Kill mutex trickery
Message-ID: <20161008140136.GG3142@twins.programming.kicks-ass.net>
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On Sat, Oct 08, 2016 at 01:58:07PM +0200, Thomas Gleixner wrote:
> 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.

Hurm,. so I thought that in general we disliked recursive locking
because it quickly turns in to a horrible mess.

Adding such primitives makes it 'easy' to use recursive locking and then
where does it stop?