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

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Message-ID: <1493837027.22125.17.camel@perches.com>
Date:   Wed, 03 May 2017 11:43:47 -0700
From:   Joe Perches <joe@...ches.com>
To:     David Howells <dhowells@...hat.com>
Cc:     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

On Wed, 2017-05-03 at 19:37 +0100, David Howells wrote:
> 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.

Huh?  How could that happen?

krealloc must always use a temporary.
If krealloc returns NULL, the original allocation is kept.

> > 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