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Date:   Mon, 27 Sep 2021 13:03:17 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Muneendra Kumar <muneendra.kumar@broadcom.com>,
        Tejun Heo <tj@kernel.org>,
        Himanshu Madhani <himanshu.madhani@oracle.com>
Subject: Re: [PATCH] block: only allocate blkcg->fc_app_id when starting to
 use it
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On Sun, Sep 26, 2021 at 03:55:07PM +0800, Ming Lei wrote:
> There isn't such sync for strlcpy, blkcg_set_fc_appid is called from
> sysfs attribute write, and cgroup_id is specified on the write buffer,
> so cmpxchg is needed. Also the comment in blkcg_set_fc_appid() explained
> that:
> 
>         /*
>          * There is a slight race condition on setting the appid.
>          * Worst case an I/O may not find the right id.
>          * This is no different from the I/O we let pass while obtaining
>          * the vmid from the fabric.
>          * Adding the overhead of a lock is not necessary.
>          */

But it isn't really a good idea.  If you have e.g. IDs with a shared
prefix this will lead to incorrect results.  I think we need a proper
locking or RCU scheme here, which should also give you the dynamic
allocation and freeing for free.