linux-kernel - Re: [PATCH] fs: load FS modules before pinning the mountpoint

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Date:	Thu, 13 Nov 2014 14:43:03 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	linux-fsdevel@...r.kernel.org, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: load FS modules before pinning the mountpoint

On Thu, Nov 13, 2014 at 03:14:33PM +0100, David Herrmann wrote:
> Several pseudo file-systems create default mountpoints in /sys/fs/ or
> /sys/kernel/, including:
>     /sys/kernel/debug
>     /sys/fs/mqueue
>     /sys/fs/cgroup
>     /sys/fs/pstore
> 
> If the given pseudo file-system is available as a module, user-space can
> rely on mount(2) to load the module on-demand (assuming MODULE_ALIAS_FS).
> However, in case the default mount-point is created by the file-system
> itself, mount(2) will fail as it cannot pin the mountpoint.
> 
> This patch makes mount(2) load any requested file-systems before pinning
> the target mount-point. This way, a file-system module can create default
> mount-points in the module instead of creating them statically in the
> core kernel.

I'm not taking that until I see a module that would attempt to do such
mountpoint creation *and* managed to get it race-free.

Besides, I really don't like the way you do this lookup only to never use
the result other than for pinning the damn module.  But that's secondary -
the main issue is with the amount of fun races in the whole "let module
create that default mountpoint" approach.
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