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    SubjectRe: [PATCH] [RFC] CFQ: simplify radix tree lookup in cfq_cic_lookup()
    On Wed, Aug 03, 2011 at 10:14:32PM +0200, Paul Bolle wrote:
    > 0) Not tested and not signed-off.
    >
    > 1) This is to see whether I understand the cfqd->cic_index usage (at
    > least, part of it).
    >
    > 2) If the lookup of a cic in the radix tree turned up a "dead" cic, then
    > that cic will be dropped. There's no reason to again try to lookup that
    > cic: that lookup should return NULL. (If it doesn't return NULL, we seem
    > to be in trouble.) So there's no need for a do {[...]} while (1) loop
    > and this code can be simplified a little.
    >
    > 3) Does this make sense?


    When a request queue exits, cfq_exit_queue(), it will free up the
    associated cic_index (ida_remove(&cic_index_ida, cfqd->cic_index)). All
    the cic which are on the request queue will be marked as dead. Now this
    cic_index is up for grab and can be re-allocated to a different request
    queue.

    Now if the same process does IO to this new queue we same cic_index as
    old request queue, then it should find the dead key and drop it and
    then allocated a new cic.

    So it does sound that there can not be more than one dead key associated
    with a cic_index at a time in ioc tree.

    But keeping current code does not harm.

    Thanks
    Vivek

    >
    >
    > Paul Bolle
    > ---
    > block/cfq-iosched.c | 28 +++++++++++++---------------
    > 1 files changed, 13 insertions(+), 15 deletions(-)
    >
    > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    > index 1f96ad6..0d33d8c 100644
    > --- a/block/cfq-iosched.c
    > +++ b/block/cfq-iosched.c
    > @@ -3120,22 +3120,20 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
    > return cic;
    > }
    >
    > - do {
    > - cic = radix_tree_lookup(&ioc->radix_root, cfqd->cic_index);
    > - rcu_read_unlock();
    > - if (!cic)
    > - break;
    > - if (unlikely(cic->key != cfqd)) {
    > - cfq_drop_dead_cic(cfqd, ioc, cic);
    > - rcu_read_lock();
    > - continue;
    > - }
    > + cic = radix_tree_lookup(&ioc->radix_root, cfqd->cic_index);
    > + rcu_read_unlock();
    >
    > - spin_lock_irqsave(&ioc->lock, flags);
    > - rcu_assign_pointer(ioc->ioc_data, cic);
    > - spin_unlock_irqrestore(&ioc->lock, flags);
    > - break;
    > - } while (1);
    > + if (!cic)
    > + return NULL;
    > +
    > + if (unlikely(cic->key != cfqd)) {
    > + cfq_drop_dead_cic(cfqd, ioc, cic);
    > + return NULL;
    > + }
    > +
    > + spin_lock_irqsave(&ioc->lock, flags);
    > + rcu_assign_pointer(ioc->ioc_data, cic);
    > + spin_unlock_irqrestore(&ioc->lock, flags);
    >
    > return cic;
    > }
    > --
    > 1.7.6
    >
    > --
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