Re: recursive spinlocks. Shoot.

[Davide Libenzi]

On Sun, 18 May 2003, Peter T. Breuer wrote:

> In article <20030518182010$[e-mail]> you wrote:
> > On Sun, 18 May 2003, Peter T. Breuer wrote:
> >> Here's a before-breakfast implementation of a recursive spinlock. That
>
> > A looong time ago I gave to someone a recursive spinlock implementation
> > that they integrated in the USB code. I don't see it in the latest
> > kernels, so I have to guess that they found a better solution to do their
> > things. I'm biased to say that it must not be necessary to have the thing
> > if you structure your code correctly.
>
> Well, you can get rid of anything that way. The question is if the
> interface is an appropriate one to use or not - i.e. if it makes for
> better code in general, or if it make errors of programming less
> likely.
>
> I would argue that the latter is undoubtedly true - merely that
> userspace flock/fcntl works that way would argue for it, but there
> are a couple of other reasons too.
>
> Going against is the point that it may be slower.  Can you dig out your
> implementation and show me it?  I wasn't going for assembler in my hasty
> example.  I just wanted to establish that it's easy, so that it becomes
> known that its easy, and folks therefore aren't afraid of it.  That both
> you and I have had to write it implies that it's not obvious code to
> everyone.

It was something like this. The real code should be in 2.2.x I believe.
The problem at the time was nested ( recursive ) spinlocks.

typedef struct s_nestlock {
	spinlock_t lock;
	void *uniq;
	atomic_t count;
} nestlock_t;

#define nestlock_init(snl) \
	do { \
		spin_lock_init(&(snl)->lock); \
		(snl)->uniq = NULL; \
		atomic_set(&(snl)->count, 0); \
	} while (0)

#define nestlock_irqlock(snl, flags) \
	do { \
		if ((snl)->uniq == current) { \
			atomic_inc(&(snl)->count); \
			flags = 0; /* No warnings */ \
		} else { \
			spin_lock_irqsave(&(snl)->lock, flags); \
			atomic_inc(&(snl)->count); \
			(snl)->uniq = current; \
		} \
	} while (0)

#define nestlock_irqunlock(snl, flags) \
	do { \
		if (atomic_dec_and_test(&(snl)->count)) { \
			(snl)->uniq = NULL; \
			spin_unlock_irqrestore(&(snl)->lock, flags); \
		} \
	} while (0)

#define nestlock_lock(snl) \
	do { \
		if ((snl)->uniq == current) { \
			atomic_inc(&(snl)->count); \
		} else { \
			spin_lock(&(snl)->lock); \
			atomic_inc(&(snl)->count); \
			(snl)->uniq = current; \
		} \
	} while (0)

#define nestlock_unlock(snl) \
	do { \
		if (atomic_dec_and_test(&(snl)->count)) { \
			(snl)->uniq = NULL; \
			spin_unlock(&(snl)->lock); \
		} \
	} while (0)

- Davide