Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

peterz
2020-09-18T10:07:59Z

On Fri, Sep 18, 2020 at 12:04:32PM +0200, peterz@infradead.org wrote:
> On Fri, Sep 18, 2020 at 12:01:12PM +0200, peterz@infradead.org wrote:
> > @@ -198,7 +198,9 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
> > */
> > static bool readers_active_check(struct percpu_rw_semaphore *sem)
> > {
> > - if (per_cpu_sum(*sem->read_count) != 0)
> > + u64 sum = per_cpu_sum(*(u64 *)sem->read_count);
> > +
> > + if (sum + (sum >> 32))
>
> That obviously wants to be:
>
> if ((u32)(sum + (sum >> 32)))
>
> > return false;
> >
> > /*

I suppose an alternative way of writing that would be something like:

union {
u64 sum;
struct {
u32 a, b;
};
} var;

var.sum = per_cpu_sum(*(u64 *)sem->read_count);

if (var.a + var.b)
return false;

which is more verbose, but perhaps easier to read.