On Tue, 24 Sep 2013 14:38:21 +0200
Peter Zijlstra <[email protected]> wrote:
> +#define cpuhp_writer_wait(cond)
> \
> +do { \
> + for (;;) { \
> + set_current_state(TASK_UNINTERRUPTIBLE); \
> + if (cond) \
> + break; \
> + schedule(); \
> + } \
> + __set_current_state(TASK_RUNNING); \
> +} while (0)
> +
> +void __get_online_cpus(void)
The above really needs a comment about how it is used. Otherwise, I can
envision someone calling this as "oh I can use this when I'm in a
preempt disable section", and the comment below for the
preempt_enable_no_resched() will no longer be true.
-- Steve
> {
> - if (cpu_hotplug.active_writer == current)
> + if (cpuhp_writer_task == current)
> return;
> - mutex_lock(&cpu_hotplug.lock);
>
> - if (WARN_ON(!cpu_hotplug.refcount))
> - cpu_hotplug.refcount++; /* try to fix things up */
> + atomic_inc(&cpuhp_waitcount);
> +
> + /*
> + * We either call schedule() in the wait, or we'll fall through
> + * and reschedule on the preempt_enable() in get_online_cpus().
> + */
> + preempt_enable_no_resched();
> + wait_event(cpuhp_wq, !__cpuhp_writer);
> + preempt_disable();
> +
> + /*
> + * It would be possible for cpu_hotplug_done() to complete before
> + * the atomic_inc() above; in which case there is no writer waiting
> + * and doing a wakeup would be BAD (tm).
> + *
> + * If however we still observe cpuhp_writer_task here we know
> + * cpu_hotplug_done() is currently stuck waiting for cpuhp_waitcount.
> + */
> + if (atomic_dec_and_test(&cpuhp_waitcount) && cpuhp_writer_task)
> + cpuhp_writer_wake();
> +}
> +EXPORT_SYMBOL_GPL(__get_online_cpus);
>
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