Re: [PATCH 1/1] prevent double open(O_RDWR) on raid creation

[Harald Hoyer <harald@redhat.com>]

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On 04/29/2013 08:11 AM, NeilBrown wrote:
> On Mon, 29 Apr 2013 07:33:21 +0200 Harald Hoyer <harald@redhat.com> wrote:
> 
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>> On 04/29/2013 02:57 AM, NeilBrown wrote:
>>> On Thu, 11 Apr 2013 15:18:33 +0200 Jes.Sorensen@redhat.com wrote:
>>> 
>>>> From: Harald Hoyer <harald@redhat.com>
>>>> 
>>>> This does not trigger the udev inotify twice and saves a lot of blk
>>>> I/O for the raid members.
>>>> 
>>>> Also fixes: https://bugzilla.redhat.com/show_bug.cgi?id=947815
>>>> 
>>>> Signed-off-by: Harald Hoyer <harald@redhat.com> Signed-off-by: Jes 
>>>> Sorensen <Jes.Sorensen@redhat.com>
>>> 
>>> (Sorry for delays.  Thanks for reminders).
>>> 
>>> That patch seems to make sense, but the description above is awfully
>>> thin.
>>> 
>>> Why is double-open a problem exactly?  What does it make udev do?  And
>>> how does that related to ID_FS_TYPE being wrong as mentioned in the
>>> bugzilla entry.
>>> 
>>> NeilBrown
>>> 
> 
>> udevd with watch enabled (inotify on /dev/sd*) gets triggered on close(),
>> when you opened it writeable. So, if you double open() and udev wakes up
>> from the first close(), not all information are written to disk yet, it
>> will not get the ID_FS_TYPE.
> 
>> Seems like the second close() does not trigger an inotify sometimes, so
>> it is missing afterwards all the time.
> 
>> Watch via inotify is just a lazy workaround, so we don't have to modify
>> every tool to emit a "change" uevent, after they changed the disk.
> 
> So udev have a "lazy workaround" so that other programs don't need to
> trigger a change, and as a result, I need to add some special code to
> mdadm. Doesn't seem like I'm getting any advantage out of this laziness.
> 
> How about when udev gets an inotify for a block device, it first checks 
> that it can open it O_EXCL.  If not, it doesn't generate the change event. 
> That seems like the laziest option to me :-)

We cannot open with O_EXCL, because the device can be mounted, and O_EXCL
would fail there.

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