Hello,
On Tue, Feb 28, 2017 at 03:51:27PM -0800, Tahsin Erdogan wrote:
> On Tue, Feb 28, 2017 at 2:47 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> >> + if (!blkcg_policy_enabled(q, pol)) {
> >> + ret = -EOPNOTSUPP;
> >> + goto fail;
> >
> > Pulling this out of the queue_lock doesn't seem safe to me. This
> > function may end up calling into callbacks of disabled policies this
> > way.
>
> I will move this to within the lock. To make things safe, I am also
> thinking of rechecking both blkcg_policy_enabled() and
> blk_queue_bypass() after reacquiring the locks in each iteration.
>
> >> + parent = blkcg_parent(blkcg);
> >> + while (parent && !__blkg_lookup(parent, q, false)) {
> >> + pos = parent;
> >> + parent = blkcg_parent(parent);
> >> + }
> >
> > Hmm... how about adding @new_blkg to blkg_lookup_create() and calling
> > it with non-NULL @new_blkg until it succeeds? Wouldn't that be
> > simpler?
> >
> >> +
> >> + new_blkg = blkg_alloc(pos, q, GFP_KERNEL);
>
> The challenge with that approach is creating a new_blkg with the right
> blkcg before passing to blkg_lookup_create(). blkg_lookup_create()
> walks down the hierarchy and will try to fill the first missing entry
> and the preallocated new_blkg must have been created with the right
> blkcg (feel free to send a code fragment if you think I am
> misunderstanding the suggestion).
Ah, indeed, but we can break out allocation of blkg and its
initialization, right? It's a bit more work but then we'd be able to
do something like.
retry:
new_blkg = alloc;
lock;
sanity checks;
blkg = blkg_lookup_and_create(..., new_blkg);
if (!blkg) {
unlock;
goto retry;
}
Thanks.
--
tejun
