linux-kernel - Re: Can ovl_drop_write() be called earlier in ovl_dentry

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Date:	Mon, 01 Jun 2015 16:53:49 +0100
From:	David Howells <dhowells@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>
Subject: Re: Can ovl_drop_write() be called earlier in ovl_dentry_open()

Miklos Szeredi <miklos@...redi.hu> wrote:

> >> Hmm,  that could result in a race where remount r/o of upper fs comes
> >> in between copy-up and vfs_open() so copy-up succeeds but the actual
> >> open fails.  It's harmless, though, and not  very likely.  So I guess
> >> your patch is OK.
> >
> > That race is there anyway if there's no copy up, right?
> 
> No.  The race I'm talking about is that with your patch it's possible
> that the file will be copied up, but open will return -EROFS.

Ah, I see what you're getting at.

> Without your patch, that is not possible since holding write counter
> for the mnt over both the copy-up and the open ensures that the
> filesystem cannot become read-only in the middle.
> 
> So your patch changes behavior, but the new behavior is acceptable,
> because there's no major change in semantics (it should only be
> detectable by the increased disk usage in the rare case of the failed
> open).

Okay.

David
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