On 09/24, Peter Zijlstra wrote:
>
> So now we drop from a no memory barriers fast path, into a memory
> barrier 'slow' path into blocking.
Cough... can't understand the above ;) In fact I can't understand
the patch... see below. But in any case, afaics the fast path
needs mb() unless you add another synchronize_sched() into
cpu_hotplug_done().
> +static inline void get_online_cpus(void)
> +{
> + might_sleep();
> +
> + /* Support reader-in-reader recursion */
> + if (current->cpuhp_ref++) {
> + barrier();
> + return;
> + }
> +
> + preempt_disable();
> + if (likely(!__cpuhp_writer))
> + __this_cpu_inc(__cpuhp_refcount);
mb() to ensure the reader can't miss, say, a STORE done inside
the cpu_hotplug_begin/end section.
put_online_cpus() needs mb() as well.
> +void __get_online_cpus(void)
> +{
> + if (__cpuhp_writer == 1) {
> + /* See __srcu_read_lock() */
> + __this_cpu_inc(__cpuhp_refcount);
> + smp_mb();
> + __this_cpu_inc(cpuhp_seq);
> + return;
> + }
OK, cpuhp_seq should guarantee cpuhp_readers_active_check() gets
the "stable" numbers. Looks suspicious... but lets assume this
works.
However, I do not see how "__cpuhp_writer == 1" can work, please
see below.
> + /*
> + * XXX list_empty_careful(&cpuhp_readers.task_list) ?
> + */
> + if (atomic_dec_and_test(&cpuhp_waitcount))
> + wake_up_all(&cpuhp_writer);
Same problem as in previous version. __get_online_cpus() succeeds
without incrementing __cpuhp_refcount. "goto start" can't help
afaics.
> void cpu_hotplug_begin(void)
> {
> - cpu_hotplug.active_writer = current;
> + unsigned int count = 0;
> + int cpu;
>
> - for (;;) {
> - mutex_lock(&cpu_hotplug.lock);
> - if (likely(!cpu_hotplug.refcount))
> - break;
> - __set_current_state(TASK_UNINTERRUPTIBLE);
> - mutex_unlock(&cpu_hotplug.lock);
> - schedule();
> - }
> + lockdep_assert_held(&cpu_add_remove_lock);
> +
> + /* allow reader-in-writer recursion */
> + current->cpuhp_ref++;
> +
> + /* make readers take the slow path */
> + __cpuhp_writer = 1;
> +
> + /* See percpu_down_write() */
> + synchronize_sched();
Suppose there are no readers at this point,
> +
> + /* make readers block */
> + __cpuhp_writer = 2;
> +
> + /* Wait for all readers to go away */
> + wait_event(cpuhp_writer, cpuhp_readers_active_check());
So wait_event() "quickly" returns.
Now. Why the new reader should see __cpuhp_writer = 2 ? It can
still see it == 1, and take that "if (__cpuhp_writer == 1)" path
above.
Oleg.
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