Re: [PATCH 43/78] Fixup device-mapper 'cookie' handling

[Hannes Reinecke <hare@suse.de>]

On 03/25/2015 05:30 PM, Benjamin Marzinski wrote:
> On Mon, Mar 16, 2015 at 01:36:30PM +0100, Hannes Reinecke wrote:
>> device-mapper has a 'cookie', which is inserted with the ioctl
>> for modifying device-mapper devices.
>> It is used as a synchronization point between udev and any other
>> applications to notify the latter when udev has finished
>> processing the event.
>> Originally multipath would only use a single cookie for every
>> transaction, and wait for that cookie at the end of the program.
>> Which works well if you only have one transaction, but for several
>> (like calling 'multipath') it will actually overwrite the cookie
>> and fail to wait for earlier events.
> 
> That shouldn't be happening.  Looking at the dm_task_set_cookie code
> 
>         if (*cookie) {
>                 if (!_get_cookie_sem(*cookie, &semid))
>                         goto_bad;
>         } else if (!_udev_notify_sem_create(cookie, &semid))
>                 goto_bad;
> 
> The first time we use that cookie, it should be zero, so a new semaphore
> will be created, and the id will be returned to us.  Subsequent calls to
> dm_task_set_cookie with the same cookie (which is now non-zero) should
> just be using the same semaphore, allowing you you wait just one time on
> all the actions linked to that cookie.  This should be more efficient
> than having to wait on each action (since udev can complete them in the
> background as we go on).
> 
> If there really are cookies that are not waited for, you should be able
> to see that by running
> 
> # dmsetup udevcookies
> 
> After running the multipath command.  Cookies won't go away until
> someone waits for them. If there are cookies listed there, then my guess
> would be that something is resetting conf->cookie to 0 after our first
> call to dm_task_set_cookie.  If there aren't, then it would appear that
> something else, instead of not waiting for the cookies, is causing
> device mapper to fail back to manual device node creation.
> 
> If you can verify that you are passing the cookie value that you get
> back from the first call to dm_task_set_cookie() into sebsequent calls,
> and you still are seeing non-watited for cookies with "dmsetup
> udevcookies" then that sounds to me like a problem in device-mapper, and
> we should check that out.
> 
Hmm. It _might_ have been caused by multipathd not properly
synchronized with udev (the original service files didn't have a
dependency on udev), hence the call to dm_udev_set_sync_support()
would cause device-mapper to switch off the udev fallback.

Speaking of nasty side-effects ...
Can't we have a proper warning in dm_udev_set_sync_support(),
alerting us that the system will not behave as expected?

But with that resolved it might be that indeed the patch is not
required. I'll check.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@suse.de			               +49 911 74053 688
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