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

Pavel Shilovsky piastry at etersoft.ru
Thu Feb 7 10:00:13 CST 2013

2013/2/7 J. Bruce Fields <bfields at fieldses.org>:
> On Thu, Feb 07, 2013 at 06:32:38PM +0400, Pavel Shilovsky wrote:
>> 2013/2/7 J. Bruce Fields <bfields at fieldses.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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