On Fri, Dec 28, 2012 at 12:35:43PM -0800, Tejun Heo wrote:
> Instead of holding blkcg->lock while walking ->blkg_list and executing
> prfill(), RCU walk ->blkg_list and hold the blkg's queue lock while
> executing prfill(). This makes prfill() implementations easier as
> stats are mostly protected by queue lock.
>
> This will be used to implement hierarchical stats.
>
Hi Tejun,
I think dropping blkcg->lock might be a problem. Using RCU we have made
sure that blkg and q are around. But what about blkg->q.backing_dev_info.dev.
We can follow bdi->dev pointer in blkg_dev_name(). I am not sure if we
ever clear it from q or not when device goes away.
Thanks
Vivek
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
> block/blk-cgroup.c | 14 +++++++++-----
> 1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index a1a4b97..22f75d1 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -504,8 +504,9 @@ static const char *blkg_dev_name(struct blkcg_gq *blkg)
> *
> * This function invokes @prfill on each blkg of @blkcg if pd for the
> * policy specified by @pol exists. @prfill is invoked with @sf, the
> - * policy data and @data. If @show_total is %true, the sum of the return
> - * values from @prfill is printed with "Total" label at the end.
> + * policy data and @data and the matching queue lock held. If @show_total
> + * is %true, the sum of the return values from @prfill is printed with
> + * "Total" label at the end.
> *
> * This is to be used to construct print functions for
> * cftype->read_seq_string method.
> @@ -520,11 +521,14 @@ void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
> struct hlist_node *n;
> u64 total = 0;
>
> - spin_lock_irq(&blkcg->lock);
> - hlist_for_each_entry(blkg, n, &blkcg->blkg_list, blkcg_node)
> + rcu_read_lock();
> + hlist_for_each_entry_rcu(blkg, n, &blkcg->blkg_list, blkcg_node) {
> + spin_lock_irq(blkg->q->queue_lock);
> if (blkcg_policy_enabled(blkg->q, pol))
> total += prfill(sf, blkg->pd[pol->plid], data);
> - spin_unlock_irq(&blkcg->lock);
> + spin_unlock_irq(blkg->q->queue_lock);
> + }
> + rcu_read_unlock();
>
> if (show_total)
> seq_printf(sf, "Total %llu\n", (unsigned long long)total);
> --
> 1.8.0.2
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