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    SubjectRe: [rfc] "fair" rw spinlocks
    Linus Torvalds <torvalds@linux-foundation.org> writes:

    > On Mon, 30 Nov 2009, Thomas Gleixner wrote:
    >>
    >> I'm aware of that. The number of places where we read_lock
    >> tasklist_lock is 79 in 36 files right now. That's not a horrible task
    >> to go through them one by one and do a case by case conversion with a
    >> proper changelog. That would only leave the write_lock sites.
    >
    > The write_lock sites should be fine, since just changing them to a
    > spinlock should be 100% semantically equivalent - except for the lack of
    > interrupt disable. And the lack of interrupt disable will result in a nice
    > big deadlock if some interrupt really does take the spinlock, which is
    > much easier to debug than a subtle race that would get the wrong read
    > value.
    >
    >> We can then either do the rw_lock to spin_lock conversion or keep the
    >> rw_lock which has no readers anymore and behaves like a spinlock for a
    >> transition time so reverts of one of the read_lock -> rcu patches
    >> could be done to debug stuff.
    >
    > So as per the above, I wouldn't worry about the write lockers. Might as
    > well change it to a spinlock, since that's what it will act as. It's not
    > as if there is any chance that the spinlock code is subtly buggy.
    >
    > So the only reason to keep it as a rwlock would be if you decide to do the
    > read-locked cases one by one, and don't end up with all of them converted.
    > Which is a reasonable strategy too, of course. We don't _have_ to convert
    > them all - if the main problem is some starvation issue, it's sufficient
    > to convert just the main read-lock cases so that writers never get
    > starved.
    >
    > But converting it all would be nice, because that whole
    >
    > write_lock_irq(&tasklist_lock);
    >
    > to
    >
    > spin_lock(&tasklist_lock);
    >
    > conversion would likely be a measurable performance win. Both because
    > spinlocks are fundamentally faster (no atomic on unlock), and because you
    > get rid of the irq disable/enable. But in order to get there, you'd have
    > to convert _all_ the read-lockers, so you'd miss the opportunity to only
    > convert the easy cases.


    Atomically sending signal to every member of a process group, is the
    big fly in the ointment I am aware of. Last time I looked I could
    not see how to convert it rcu.

    Fundamentally: "kill -KILL -pgrp" should be usable to kill all of
    the processes in a process group, and "kill -KILL -1" should be usable
    to kill everything except the sender and init. Something I have seen
    in shutdown scripts on more than one occasion.

    This is a subtle in the sense that it won't show up in simple tests if
    you get it wrong.

    This is a pain because we occasionally signal a process group from
    interrupt context.

    The trouble as I recall is how to ensure new processes see the signal.

    Eric


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