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    SubjectRe: [PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes
    On Thu, 18 Jan 2018 10:53:37 +0900
    Byungchul Park <byungchul.park@lge.com> 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:
    >
    > This context
    > ------------
    > while (READ_ONCE(console_waiter)) /* Wait for the event */
    > cpu_relax();
    >
    > Another context
    > ---------------
    > WRITE_ONCE(console_waiter, false); /* Event */

    I disagree. It is like a spinlock. You can say a spinlock() that is
    blocked is also waiting for an event. That event being the owner does a
    spin_unlock().

    >
    > 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:
    >
    > A context
    > ---------
    > lock_map_acquire(wait); /* Or lock_map_acquire_read(wait) */
    > /* Read one is better though.. */
    >
    > /* A section, we suspect, a wait for an event might happen. */
    > ...
    > lock_map_release(wait);
    >
    >
    > The place actually doing the wait
    > ---------------------------------
    > lock_map_acquire(wait);
    > lock_map_acquire(wait);
    >
    > wait_for_event(wait); /* Actually do the wait */
    >
    > You can see a simple example of how to use them by searching
    > kernel/cpu.c with "lock_acquire" and "wait_for_completion".
    >
    > However, as I said, if you suspect that cpu_relax() includes
    > the wait, then it's ok to leave it. Otherwise, I think it
    > would be better to change it in the way I showed you above.

    I find your way confusing. I'm simulating a spinlock not a wait for
    completion. A wait for completion usually initiates something then
    waits for it to complete. This is trying to get into a critical area
    but another task is currently in it. It's simulating a spinlock as far
    as I can see.

    -- Steve

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