Re: [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes

[Byungchul Park]

On 1/18/2018 10:53 AM, Byungchul Park wrote:
> Hello,
>
> This is a thing simulating a wait for an event e.g.
> wait_for_completion() doing spinning instead of sleep, rather
> than a spinlock. I mean:
>
>  ÂÂ This context
>  ÂÂ ------------
>  ÂÂ while (READ_ONCE(console_waiter)) /* Wait for the event */
>  ÂÂÂÂÂ cpu_relax();
>
>  ÂÂ Another context
>  ÂÂ ---------------
>  ÂÂ WRITE_ONCE(console_waiter, false); /* Event */
>
> That's why I said this's the exact case of cross-release. Anyway
> without cross-release, we usually use typical acquire/release
> pairs to cover a wait for an event in the following way:
>
>  ÂÂ A context
>  ÂÂ ---------
>  ÂÂ lock_map_acquire(wait); /* Or lock_map_acquire_read(wait) */
>  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /* Read one is better though..ÂÂÂ */
>
>  ÂÂ /* A section, we suspect, a wait for an event might happen. */
>  ÂÂ ...
>  ÂÂ lock_map_release(wait);
>
>
>  ÂÂ The place actually doing the wait
>  ÂÂ ---------------------------------
>  ÂÂ lock_map_acquire(wait);
>  ÂÂ lock_map_acquire(wait);
      ^
      lock_map_release(wait);

--
Thanks,
Byungchul