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Date:   Thu, 20 Jan 2022 15:20:14 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Jan Kara <jack@suse.cz>
Cc:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Dan Schatzberg <schatzberg.dan@gmail.com>,
        kernel test robot <oliver.sang@intel.com>,
        Jan Stancek <jstancek@redhat.com>,
        linux-block <linux-block@vger.kernel.org>
Subject: Re: [PATCH v2 2/2] loop: use task_work for autoclear operation
Message-ID: <20220120142014.GA11879@lst.de>
References: <969f764d-0e0f-6c64-de72-ecfee30bdcf7@I-love.SAKURA.ne.jp> <bcaf38e6-055e-0d83-fd1d-cb7c0c649372@I-love.SAKURA.ne.jp> <20220110103057.h775jv2br2xr2l5k@quack3.lan> <fc15d4a1-a9d2-1a26-71dc-827b0445d957@I-love.SAKURA.ne.jp> <20220110134234.qebxn5gghqupsc7t@quack3.lan> <d1ca4fa4-ac3e-1354-3d94-1bf55f2000a9@I-love.SAKURA.ne.jp> <20220112131615.qsdxx6r7xvnvlwgx@quack3.lan> <20220113104424.u6fj3z2zd34ohthc@quack3.lan>
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On Thu, Jan 13, 2022 at 11:44:24AM +0100, Jan Kara wrote:
> Maybe the most disputable thing in this locking chain seems to be splicing
> from sysfs files. That does not seem terribly useful and due to special
> locking and behavior of sysfs files it allows for creating interesting lock
> dependencies. OTOH maybe there is someone out there who (possibly
> inadvertedly through some library) ends up using splice on sysfs files so
> chances for userspace breakage, if we disable splice for sysfs, would be
> non-negligible. Hum, tough.

People were using sendfile on sysfs files, that is why support for this
got added back after it was removed for a while as part of the set_fs()
removal.

The real question for me is why do we need freeing and writer counts on
sysfs or any other pure in-memory file system to start with?