linux-kernel - Re: [PATCH -next V3] ubi: fix race condition between ctrl_cdev_ioctl and ubi_cdev

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Date:   Thu, 30 Dec 2021 22:32:27 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     chengzhihao1 <chengzhihao1@...wei.com>
Cc:     libaokun <libaokun1@...wei.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        yukuai3 <yukuai3@...wei.com>, Hulk Robot <hulkci@...wei.com>
Subject: Re: [PATCH -next V3] ubi: fix race condition between
 ctrl_cdev_ioctl and ubi_cdev_ioctl

----- Ursprüngliche Mail -----
> Von: "chengzhihao1" <chengzhihao1@...wei.com>
> kernfs_drain() will wait 'root->deactivate_waitq' if
> atomic_read(&kn->active) not equals to KN_DEACTIVATED_BIAS.
> 
> The UBI seq_show callback is invoked with avtive cnt taken:
> vfs_read
>   kernfs_fop_read_iter
>     seq_read_iter
>       m->op->start (kernfs_seq_start)   // kernfs_get_active(of->kn)
>       kernfs_seq_show
>         dev_attribute_show [ubi]
>       m->op->stop (kernfs_seq_stop)     // kernfs_put_active(of->kn)
> 
> The kernfs_drain() is stuck at wait_event() until sysfs reading
> finished, in my local test.

You are right. This means UBI does this extra check in vain.
Maybe even since ever.

Thanks,
//richard