linux-kernel - Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked

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Message-ID: <20230926094130.GC13806@lst.de>
Date:   Tue, 26 Sep 2023 11:41:30 +0200
From:   Christoph Hellwig <hch@....de>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        Christian Brauner <brauner@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        Tejun Heo <tj@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Damien Le Moal <dlemoal@...nel.org>,
        Naohiro Aota <naohiro.aota@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-hardening@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked_super

On Thu, Sep 14, 2023 at 06:38:43AM +0100, Al Viro wrote:
> It's worse - look at the rationale for 2c18a63b760a "super: wait until
> we passed kill super".  Basically, "don't remove from the lists
> until after block device closing".  IOW, we have

As of this stage we don't even touch anything related to block devices..

> That was *NOT* what a recent rework had done.  Block device closing had never
> been inside ->put_super() - at no point since that (closing, that is) had been
> introduced back in 0.97 ;-)  ->put_super() predates it (0.95c+).
> 
> The race is real, but the cause is not some kind of move of blkdev_put().
> Your 2ea6f68932f7 "fs: use the super_block as holder when mounting file
> systems" is where it actually came from.
> 
> Christoph, could you explain what the hell do we need that for?  It does
> create the race in question and AFAICS 2c18a63b760a (and followups trying
> to plug holes in it) had been nothing but headache.

Because it allows us to actually get from the bdev to the holder directly,
someting we've badly neeed for a while.