Re: Fix locking in input

[Manfred Spraul <manfred@colorfullife.com>]

Paul wrote:

>Pavel Machek writes:
>
>> 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.

--
	Manfred