Re: [PATCH] udev enhancements to use kernel event queue

[Paul Mackerras]

Patrick Mochel writes:

> It seems like the following should work. Would someone mind commenting on
> it?

> +/**
> + * atomic_inc_and_read - increment atomic variable and return new value
> + * @v: pointer of type atomic_t
> + * 
> + * Atomically increments @v by 1.  Note that the guaranteed
> + * useful range of an atomic_t is only 24 bits.
> + */ 
> +static inline int atomic_inc_and_read(atomic_t *v)
> +{
> +	__asm__ __volatile__(
> +		LOCK "incl %0"
> +		:"=m" (v->counter)
> +		:"m" (v->counter));
> +	return v->counter;
> +}

BZZZT.  If another CPU is also doing atomic_inc_and_read you could end
up with both calls returning the same value.

You can't do atomic_inc_and_read on 386.  You can on cpus that have
cmpxchg (e.g. later x86).  You can also on machines with load-locked
and store-conditional instructions (alpha, ppc, probably most other
RISCs).

Paul.