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    SubjectRe: [PATCH 2/3] rcu: Create rcu_sync infrastructure
    On Wed, Oct 02, 2013 at 05:49:01PM +0200, Oleg Nesterov wrote:
    > On 10/02, Peter Zijlstra wrote:
    > >
    > > From: Oleg Nesterov <oleg@redhat.com>
    >
    > Thanks! I was writing the patch, and I chose almost the same naming ;)
    >
    > > Signed-off-by: Peter Zijlstra <peterz@infradead.org>
    >
    > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    >
    > In fact I'd like to add my sob to 1/3 and 3/3 as well.
    >
    >
    > Paul, to remind, this is only the first step. I am going to send
    > the following improvements:
    >
    > 1. Add rcu_sync->exlusive. The change is simple, just we
    > need s/wait_queue_head_t/completion/ in rcu_sync_struct
    > and a couple of "if (rss->exclusive)" checks in enter/exit.
    >
    > 2. rcu_sync_enter() should return !!need_sync. This can help
    > in exclusive mode.
    >
    > 3. rcu_sync_struct needs more function pointers (perhaps we
    > should add a single rcu_sync_struct->ops pointer but this
    > is minor). See below.
    >
    > But let me repeat just in case, we should do this later.
    > And once this series is applied, I'll change percpu_rw_semaphore.

    I took this into account in my review, the upgrades would be good! ;-)

    Thanx, Paul

    > > +struct rcu_sync_struct {
    > > + int gp_state;
    > > + int gp_count;
    > > + wait_queue_head_t gp_wait;
    > > +
    > > + int cb_state;
    > > + struct rcu_head cb_head;
    > > +
    > > + void (*sync)(void);
    > > + void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
    >
    > Yes, and we also need rcu_sync_struct->barrier(). From the patch I was
    > working on:
    >
    > void rcu_sync_wait_for_callback(struct rcu_sync *sync)
    > {
    > int cb_state;
    >
    > BUG_ON(sync->gp_count);
    >
    > spin_lock_irq(&sync->state_lock);
    > if (sync->cb_state == CB_REPLAY)
    > sync->cb_state = CB_PENDING;
    > cb_state = sync->cb_state;
    > spin_unlock_irq(&sync->state_lock);
    >
    > if (cb_state != CB_IDLE) {
    > rcu_barrier_sched();
    > BUG_ON(sync->cb_state != CB_IDLE);
    > }
    > }
    >
    > It should be called if you are going to kfree the object.
    >
    > Perhaps another rcu_sync_struct->state_change(new_state) callback (set
    > by the user) makes sense too, this can help (for example) to implement
    > the array of semaphores with a single rcu_sync_struct (freeze_super).
    >
    > Thanks.
    >
    > Oleg.
    >



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