linux-kernel - Re: [PATCH 3/9] VFS: Introduce a mount context

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Message-ID: <4597.1493836661@warthog.procyon.org.uk>
Date:   Wed, 03 May 2017 19:37:41 +0100
From:   David Howells <dhowells@...hat.com>
To:     Joe Perches <joe@...ches.com>
Cc:     dhowells@...hat.com, Jeff Layton <jlayton@...chiereds.net>,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        mszeredi@...hat.com
Subject: Re: [PATCH 3/9] VFS: Introduce a mount context

Joe Perches <joe@...ches.com> wrote:

> krealloc would probably be more efficient and possible
> readable as likely there's already padding in the original
> allocation.

The problem is if krealloc() fails: you've lost all those pointers to things
you then need to free.

> Are there no locking constraints?

Generally, no, not until you do the ->mount() op.  Also remounting needs a
lock, but that's already done with the sb->s_umount lock.

However, that said, if you do:

	fd = fsopen("foofs");
	write(fd, "o foo=bar", ...);
	fsmount(fd, "/foo");

then the fsmount() and write() calls have to lock against other fsmount() and
write() calls.  I use the inode lock for this.  [Note that it probably should
be interruptible rather than just killable, but there's no primitive for that
as yet].

David