spin_lock after get_cpu_var()

spin_lock after get_cpu_var()

[Peter Teoh]

I find it quite puzzling (no where else in kernel source is this
found) that you would want to apply spin_lock() after get_cpu_var().
The fddef is already percpu, so there is no need to lock it, right?

I submitted this patch before, but got no response, just trying my
luck this time :-).

void free_fdtable_rcu(struct rcu_head *rcu)
{
        struct fdtable *fdt = container_of(rcu, struct fdtable, rcu);
        struct fdtable_defer *fddef;

        BUG_ON(!fdt);

        if (fdt->max_fds <= NR_OPEN_DEFAULT) {
                /*
                 * This fdtable is embedded in the files structure and that
                 * structure itself is getting destroyed.
                 */
                kmem_cache_free(files_cachep,
                                container_of(fdt, struct files_struct, fdtab));
                return;
        }
        if (fdt->max_fds <= (PAGE_SIZE / sizeof(struct file *))) {
                kfree(fdt->fd);
                kfree(fdt->open_fds);
                kfree(fdt);
        } else {
                fddef = &get_cpu_var(fdtable_defer_list);=============> here
                spin_lock(&fddef->lock);==========================>here
                fdt->next = fddef->next;
                fddef->next = fdt;
                /* vmallocs are handled from the workqueue context */
                schedule_work(&fddef->wq);
                spin_unlock(&fddef->lock);
                put_cpu_var(fdtable_defer_list);
        }
}

Re: spin_lock after get_cpu_var()

[Johannes Weiner]

Hi Peter,

"Peter Teoh" <htmldeveloper@gmail.com> writes:

> I find it quite puzzling (no where else in kernel source is this
> found) that you would want to apply spin_lock() after get_cpu_var().
> The fddef is already percpu, so there is no need to lock it, right?
>
> I submitted this patch before, but got no response, just trying my
> luck this time :-).

http://lkml.org/lkml/2008/3/14/208

	Hannes