linux-kernel - Re: [PATCH v2 3/8] vfs: Add O_DENYREAD/WRITE flags support for open syscall

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Message-ID: <CAKywueShbPd9b+WmJwnfwPR_vzk_atBVRBZNTf-HpS7N2CK+AA@mail.gmail.com>
Date:	Thu, 7 Feb 2013 20:00:13 +0400
From:	Pavel Shilovsky <piastry@...rsoft.ru>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-nfs@...r.kernel.org,
	wine-devel@...ehq.org
Subject: Re: [PATCH v2 3/8] vfs: Add O_DENYREAD/WRITE flags support for open syscall

2013/2/7 J. Bruce Fields <bfields@...ldses.org>:
> On Thu, Feb 07, 2013 at 06:32:38PM +0400, Pavel Shilovsky wrote:
>> 2013/2/7 J. Bruce Fields <bfields@...ldses.org>:
>> > On Thu, Feb 07, 2013 at 01:53:46PM +0400, Pavel Shilovsky wrote:
>> >> Nothing prevents it. If somebody grabbed a share mode lock on a file
>> >> before we call deny_lock_file, we simply close this file and return
>> >> -ETXTBSY.
>> >
>> > But leave the newly-created file there--ugh.
>> >
>> >> We can't grab it before atomic_open because we don't have an
>> >> inode there.
>> >
>> > If you can get the lock while still holding the directory i_mutex can't
>> > you prevent anyone else from looking up the new file until you've gotten
>> > the lock?
>> >
>>
>> Hm..., seems you are right, I missed this part:
>> mutex_lock
>> lookup_open -> atomic_open -> deny_lock_file
>> mutex_unlock
>>
>> that means that nobody can open and of course set flock on the newly
>> created file (because flock is done through file descriptor). So, it
>> should be fine to call flock after f_ops->atomic_open in atomic_open
>> function. Thanks.
>
> Whether that works may also depend on how the new dentry is set up?  If
> it's hashed before you call flock then I suppose it's already visible to
> others.

It seems it should be hashed in f_ops->atomic_open() (at least cifs
and nfs do it this way). In do_last when we do an ordinary open, we
don't hit parent i_mutex if lookup is succeeded through lookup_fast.
lookup_fast can catch newly created dentry and set it's share mode
before atomic_open codepath hits deny_lock_file.

Also, I noted that: atomic open does f_ops->atomic_open and then it
processes may_open check; if may_open fails, the file is closed and
open returns with a error code (but file is created anyway) . I think
there is no difference between this case and the situation with
deny_lock_file there.

-- 
Best regards,
Pavel Shilovsky.
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