udev hangs under high loads

[Greg KH <greg@kroah.com>]

One of my coworkers (hi Nish) is trying to get 10000 disk support tested
and working properly with linux and ran into a nasty udev issue
yesterday.  It seems that under a very high load, with a whole bunch of
hotplug events happening (for when the disks get added to the system),
udev hangs.

It hangs in the "grabbing the database lock" portion of the code (don't
have actual logs of where it hangs, will try to get that next week.)
But the interesting thing is we almost handle everything properly.  
udev creates the node, tries to write to the database.  Then the timer
expires and we report this.  After the timer expires, udev is done for.
It just sits and spins, doing a nanosleep constantly.  Have 500 of those
instances all running at once, all at a nice level of -10 is a sure way
to bring a box (even a relatively big one) down hard.

So, while I'll agree finding the root tdb locking bug is a good idea, I
wanted to point out that perhaps we should just exit udev if our timeout
expires, instead of still waiting around.  Or do you have a better
solution?

Try testing this out on your own using the scsi_debug module and adding
a few hundred disks.  It also helps if you have scsi generic support
enabled, as that creates 2 udev events for every disk created.

thanks,

greg k-h


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