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SubjectRe: recursive spinlocks. Shoot.
On Sun, 18 May 2003, Peter T. Breuer wrote:

> Here's a before-breakfast implementation of a recursive spinlock. That
> is, the same thread can "take" the spinlock repeatedly. This is crude -
> I just want to focus some attention on the issue (while I go out and
> have breakfast :'E).
> The idea is to implement trylock correctly, and then get lock and
> unlock from that via the standard algebra. lock is a loop doing
> a trylock until it succeeds. We emerge from a successful trylock
> with the lock notionally held.
> The "spinlock" is a register of the current pid, plus a recursion
> counter, with atomic access. The pid is either -1 (unset, count is
> zero) or some decent value (count is positive).
> The trylock will succeed and set the pid if it is currently unset. It
> will succeed if the pid matches ours, and increment the count of
> holders.
> Unlock just decrements the count. When we've unlocked enough times,
> somebody else can take the lock.

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.

- Davide

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