Re: [PATCH 5/8] cpu: cpu-hotplug deadlock

[Oleg Nesterov]

On 04/29, Gautham R Shenoy wrote:
>
> cpu_hotplug.mutex is basically a lock-internal lock; but by keeping it locked
> over the 'write' section (cpu_hotplug_begin/done) a lock inversion happens when
> some of the write side code calls into code that would otherwise take a
> read lock.
>
> And it so happens that read-in-write recursion is expressly permitted.
>
> Fix this by turning cpu_hotplug into a proper stand alone unfair reader/writer
> lock that allows reader-in-reader and reader-in-writer recursion.

While the patch itself is very clean and understandable, I can't understand
the changelog ;)

Could you explain what is the semantics difference? The current code allows
read-in-write recursion too.

The only difference I can see is that now cpu_hotplug_begin() doesn't rely
on cpu_add_remove_lock any longer (currently the caller must hold this lock),
but this (good) change is not documented.

>  static void cpu_hotplug_done(void)
>  {
> +	spin_lock(&cpu_hotplug.lock);
>  	cpu_hotplug.active_writer = NULL;
> -	mutex_unlock(&cpu_hotplug.lock);
> +	if (!list_empty(&cpu_hotplug.writer_queue.task_list))

waitqueue_active() ?

> +		wake_up(&cpu_hotplug.writer_queue);
> +	else
> +		wake_up_all(&cpu_hotplug.reader_queue);

Please note that wake_up() and wake_up_all() doesn't differ here, because
cpu_hotplug_begin() use prepare_to_wait(), not prepare_to_wait_exclusive().
I'd suggest to change cpu_hotplug_begin(), and use wake_up() for both
cases.

(actually, since write-locks should be very rare, perhaps we don't need
 2 wait_queues ?)

Oleg.

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