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SubjectRe: [PATCH 5/5] CFQ: use proper locking for cache of last hit cic
On 2011-06-05 18:26, Paul Bolle wrote:
> io_context.last_cic is a (single entry) cache of the last hit
> cfq_io_context ("cic").
>
> It turns out last_cic wasn't always accessed with io_context.lock held
> and under the correct RCU semantics. That meant that last_cic could be
> out of sync with the hlist it was supposed to cache, leading to hard to
> reproduce and hard to debug issues. Using proper locking makes those
> issues go away.
>
> Many thanks to Vivek Goyal, Paul McKenney, and Jens Axboe, in suggesting
> various options, looking at all the debug output I generated, etc. If we
> hadn't done all that I would have never concluded that the best way to
> solve this issue was to, yet again, read the code looking for
> problematic sections.
>
> This should finally resolve bugzilla.redhat.com/show_bug.cgi?id=577968

A few comments inline.

> Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
> ---
> block/cfq-iosched.c | 27 +++++++++++++++++++--------
> 1 files changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 39e4d01..9206ee3 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -2695,6 +2695,8 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
> struct cfq_io_context *cic)
> {
> struct io_context *ioc = cic->ioc;
> + struct cfq_io_context *last_cic;
> + unsigned long flags;
>
> list_del_init(&cic->queue_list);
>
> @@ -2704,8 +2706,13 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
> smp_wmb();
> cic->key = cfqd_dead_key(cfqd);
>
> - if (ioc->last_cic == cic)
> + spin_lock_irqsave(&ioc->lock, flags);
> + rcu_read_lock();
> + last_cic = rcu_dereference(ioc->last_cic);
> + rcu_read_unlock();
> + if (last_cic == cic)
> rcu_assign_pointer(ioc->last_cic, NULL);
> + spin_unlock_irqrestore(&ioc->lock, flags);

We don't need the ioc->lock for checking the cache, it would in fact
defeat the purpose of using RCU. But this hunk will clash with the
merged part anyway.

> @@ -3000,23 +3007,25 @@ cfq_get_queue(struct cfq_data *cfqd, bool is_sync, struct io_context *ioc,
>
> /*
> * We drop cfq io contexts lazily, so we may find a dead one.
> + *
> + * Called with ioc->lock held.
> */
> static void
> cfq_drop_dead_cic(struct cfq_data *cfqd, struct io_context *ioc,
> struct cfq_io_context *cic)
> {
> - unsigned long flags;
> + struct cfq_io_context *last_cic;
>
> WARN_ON(!list_empty(&cic->queue_list));
> BUG_ON(cic->key != cfqd_dead_key(cfqd));
>
> - spin_lock_irqsave(&ioc->lock, flags);
> -
> - BUG_ON(ioc->last_cic == cic);
> + rcu_read_lock();
> + last_cic = rcu_dereference(ioc->last_cic);
> + rcu_read_unlock();
> + BUG_ON(last_cic == cic);
>
> radix_tree_delete(&ioc->radix_root, cfqd->cic_index);
> hlist_del_rcu(&cic->cic_node);
> - spin_unlock_irqrestore(&ioc->lock, flags);
>
> cfq_cic_free(cic);

See Pauls comment on this part.

--
Jens Axboe



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