Re: [RFC] netlink: get rid of nl_table_lock

[Thomas Graf <tgraf@suug.ch>]

On 01/03/15 at 11:02am, Stephen Hemminger wrote:
> As a follow on to Thomas's patch I think this would complete the
> transistion to RCU for netlink.
> Compile tested only.
> 
> 
> 
> This patch gets rid of the reader/writer nl_table_lock and replaces it
> with exclusively using RCU for reading, and a mutex for writing.
> 
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

I like it. One thing I noticed it that it leaves a mix of
native mutex unlocks and unlocks via netlink_table_ungrab().

The Open vSwitch upcall is a pretty good real world Netlink
benchmark. I'll run the tests I have to see if this has any
unexpected side effects.

>  void netlink_table_grab(void)
> -	__acquires(nl_table_lock)
>  {
> -	might_sleep();
> -
> -	write_lock_irq(&nl_table_lock);
> -
> -	if (atomic_read(&nl_table_users)) {
> -		DECLARE_WAITQUEUE(wait, current);
> -
> -		add_wait_queue_exclusive(&nl_table_wait, &wait);
> -		for (;;) {
> -			set_current_state(TASK_UNINTERRUPTIBLE);
> -			if (atomic_read(&nl_table_users) == 0)
> -				break;
> -			write_unlock_irq(&nl_table_lock);
> -			schedule();
> -			write_lock_irq(&nl_table_lock);
> -		}
> -
> -		__set_current_state(TASK_RUNNING);
> -		remove_wait_queue(&nl_table_wait, &wait);
> -	}
> +	mutex_lock(&nl_table_mutex);
>  }

I left this untouched so far as I wasn't clear on what side effect
it would have to remove this as it does explicitely relax writers
right now.