[PATCH] fix failure when root filesystem is on nvme

[Mikulas Patocka]

On Sat, 21 Apr 2018, Keith Busch wrote:

> On Sat, Apr 21, 2018 at 09:59:48AM -0400, Mikulas Patocka wrote:
>
> > There's a bug in the nvme block device driver that causes failure when we 
> > have no initramfs and the root filesystem is directly on nvme. The driver 
> > spawns a work item nvme_reset_work() in the nvme_wq workqueue, but doesn't 
> > wait for it. The result is that the kernel attempts to mount the root 
> > filesystem before nvme_reset_work() finishes and it panics because it 
> > can't find the root device.
> > 
> > It can be fixed with this simple patch (perhaps you can come up with a 
> > better patch that uses the asynchronous probing infrastructure?)
> 
> We probe asynchronously to fix other issues.
> 
> First is that boot takes way
> too long if you have a lot of devices when probing all of them serially,
> and then certain init systems kill the probe task after a certain time,
> breaking boot for those.
> 
> Is there something we can do for your setup to have the kernel wait for
> the root partition to be available instead of givinig up after pci probe?

Are different PCI NVME devices probed concurrently by the PCI API? (I 
can't try, I have just one) If yes, then the patch that I posted should be 
OK, because it wouldn't break this concurrency.

If not, then you need to make sure that wait_for_device_probe() waits for 
the NVME probe to finish. The kernel calls wait_for_device_probe() just 
before it attempts to mount the root filesystem. I don't know which of the 
kernel frameworks would be best suited to accomplish that. Perhaps the 
simplest solution would be to increment probe_count in nvme_probe and 
decrement it when the probe work item finishes - but it is not exported 
and you'd need to create helper functions in drivers/base/dd.c to do that.

Mikulas