[dm-devel] [PATCH v4 09/20] libmultipath: functions to indicate mapping failure in /dev/shm

[Benjamin Marzinski]

On Thu, Apr 12, 2018 at 10:07:13PM +0200, Martin Wilck wrote:
> On Thu, 2018-04-12 at 13:33 -0500, Benjamin Marzinski wrote:
> > On Wed, Apr 04, 2018 at 06:16:16PM +0200, Martin Wilck wrote:
> > > Create a simple API that indicates failure to create a map for a
> > > certain WWID. This will allow multipathd to indicate to other tools
> > > (in particular, "multipath -u" during udev processing) that
> > > an attempt to create a map for a certain wwid failed.
> > > 
> > > The indicator is simply the existence of a file under
> > > /dev/shm/multipath/failed_wwids.
> > 
> > I'm a little confused about the necessity of a lock file here.  What
> > it
> > the race that you are worried about? If two processes try to create a
> > file at the same time, surely one of them will succeed. If two
> > processes
> > try to delete a file at the same time, it will get deleted.  If one
> > process is trying to create a file and one is trying to remove it,
> > the
> > outcome depends on the who wins the race. But this is true whether
> > you
> > add a lock file to make those actions atomic or not. The same goes
> > with
> > stating a file that's being created or removed. As far a I can tell,
> > this should work if you simply create an empty file without any
> > locking.
> > Are you worried about some odd errno due to a race?
> 
> My thinking was that it's generally a good thing to make file system
> operations like this atomic, and the well-tested and mature open_file()
> API was there ready to be used, thus I did.
> 
> You're probably right that the locking not strictly necessary. I don't
> think it hurts, performance-wise the impact is quite low.
> Do you require me to change this, or can we change it later?

That's fine. I don't see how it can hurt.

Reviewed-by: Benjamin Marzinski <bmarzins at redhat.com>

> 
> Martin
> 
> -- 
> Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
> SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
> HRB 21284 (AG Nürnberg)