Date: Wed, 1 Mar 2017 11:55:01 -0500
From: Tejun Heo <tj@kernel.org>
To: Tahsin Erdogan <tahsin@google.com>
Cc: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        David Rientjes <rientjes@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] blkcg: allocate struct blkcg_gq outside request queue
 spinlock
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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@kernel.org> 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