Messages in this thread | | | From | David Brownell <> | Subject | Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support | Date | Wed, 14 Nov 2007 22:50:17 -0800 |
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On Wednesday 14 November 2007, Haavard Skinnemoen wrote: > On Wed, 14 Nov 2007 00:37:57 -0800 > David Brownell <david-b@pacbell.net> wrote: > > > Although another point is related to "trivial": the data > > is being protected through an operation too trivial to be > > worth paying for any of that priority logic. > > But isn't there any way we can remove the lock from the fast path > altogether? What is it really protecting?
The integrity of the table. Entries can be added and removed (both operations being *RARE* which is good!) at any time.
> Since this is the code that runs under the lock
No, there's more than that. This is what runs under it in the hot paths, yes, but the gpio request/free paths do more work than this. (That includes direction setting, since that can be an implicit request.)
> (excluding the "extra checks" case): > > +static inline struct gpio_chip *gpio_to_chip(unsigned gpio) > +{ > + return chips[gpio / ARCH_GPIOS_PER_CHIP]; > +} > > I'd say it protects against chips being removed in the middle of the > operation. However, this comment says that chips cannot be removed > while any gpio on it is requested: > > +/* gpio_lock protects the table of chips and to gpio_chip->requested. > + * While any gpio is requested, its gpio_chip is not removable. It's > + * a raw spinlock to ensure safe access from hardirq contexts, and to > + * shrink bitbang overhead: per-bit preemption would be very wrong. > + */ > > And since we drop the lock before calling the actual get/set bit > operation, we would be screwed anyway if the chip was removed during > the call to __gpio_set_value(). So what does the lock really buy us?
The get/set bit calls are the hot paths. Locking on those paths buys us a consistent locking policy, which is obviously correct. It's consistent with the request/free paths.
But I think what you're suggesting is that the "requested" flag is effectively a long-term lock, so grabbing the spinlock on those paths is not necessary. Right?
Hmm ... that makes some sense. I hadn't started out thinking of that "requested" flag as a lock bit, but in fact that's what it ended up becoming.
Removing the spinlock from those paths -- at least in the "no extra checks case" -- would let us avoid all this flamage about whether raw spinlocks are ever OK.
I think I forsee a patch coming...
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