Regression in 3.15 on POWER8 with multipath SCSI

[Paul Mackerras]

On Mon, Jun 30, 2014 at 12:52:29PM +0200, Hannes Reinecke wrote:
> On 06/30/2014 12:30 PM, Paul Mackerras wrote:
> >I have a machine on which 3.15 usually fails to boot, and 3.14 boots
> >every time.  The machine is a POWER8 2-socket server with 20 cores
> >(thus 160 CPUs), 128GB of RAM, and 7 SCSI disks connected via a
> >hardware-RAID-capable adapter which appears as two IPR controllers
> >which are both connected to each disk.  I am booting from a disk that
> >has Fedora 20 installed on it.
> >
> >After over two weeks of bisections, I can finally point to the commits
> >that cause the problems.  The culprits are:
> >
> >3e9f1be1 dm mpath: remove process_queued_ios()
> >e8099177 dm mpath: push back requests instead of queueing
> >bcccff93 kobject: don't block for each kobject_uevent
> >
> >The interesting thing is that neither e8099177 nor bcccff93 cause
> >failures on their own, but with both commits in there are failures
> >where the system will fail to find /home on some occasions.
> >
> >With 3e9f1be1 included, the system appears to be prone to a deadlock
> >condition which typically causes the boot process to hang with this
> >message showing:
> >
> >A start job is running for Monitoring of LVM2 mirror...rogress polling
> >
> >(with a [***     ] thing before it where the asterisks move back and
> >forth).
> >
> >If I revert 63d832c3 ("dm mpath: really fix lockdep warning") ,
> >4cdd2ad7 ("dm mpath: fix lock order inconsistency in
> >multipath_ioctl"), 3e9f1be1 and bcccff93, in that order, I get a
> >kernel that will boot every time.  The first two are later commits
> >that fix some problems with 3e9f1be1 (though not the problems I am
> >seeing).
> >
> >Can anyone see any reason why e8099177 and bcccff93 would interfere
> >with each other?
> >
> It might be running afoul with the 'cookie' mechanism.
> Device-mapper is using inserting a 'cookie' with the ioctl, and listens to
> any event containing the cookie to ensure udev has finished processing that
> device and hence the device node is accessible. Added to this is the problem
> that we don't have any good means of detecting any changes to device-mapper
> devices.

How does that relate to e8099177?  Did e8099177 introduce this cookie
mechanism?  If not, what is it about e8099177 that makes the async
processing problematic?

Paul.