linux-kernel - Re: [PATCH] blkcg: allocate struct blkcg

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Date:   Tue, 28 Feb 2017 15:51:27 -0800
From:   Tahsin Erdogan <tahsin@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blkcg: allocate struct blkcg_gq outside request queue spinlock

On Tue, Feb 28, 2017 at 2:47 PM, Tejun Heo <tj@...nel.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).