Re: Fix locking in input

[Pavel Machek]

Hi!

> >>input uses "volatile signed char" as a shared variable between normal
> >>and interrupt threads (look at _sendbyte()). Thats bad idea, this
> >>switches it to atomic_t.
> >
> >This change looks unnecessary to me - we aren't trying to increment or
> >decrement the variable, just set it and read it.  Reading and writing
> >individual bytes is atomic on any platform we care about.
> > 
> >
> I think one platform (early ARM?) cannot access bytes directly, and 
> implement the access with read 16-bit, change 8-bit, write back 16 bit. 
> Reading/writing pointers or longs is atomic.
> 
> Pavel: Do you know that atomic_set and atomic_read aren't memory barriers?
> I.e.
> 
> -	psmouse->ack = 0;
> +	atomic_set(&psmouse->ack, 0);
> 	psmouse->acking = 1;
> 
> It's not guaranteed that all cpus will see psmouse->ack=0 before 
> psmouse->acking=1. And adding the required memory barriers usually makes 
> the code completely unreadable, thus I usually give up and switch to a 
> spinlock.

Would simply adding mb() there fix it? It is not performance-critical
in any way...
								Pavel

-- 
When do you have a heart between your knees?
[Johanka's followup: and *two* hearts?]
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/