Re: Regression in 3.15 on POWER8 with multipath SCSI

On Mon, Jun 30 2014 at  6:30am -0400,
Paul Mackerras [off-list ref] 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?

No, not seeing any obvious relation.

But even though you listed e8099177 as a culprit you didn't list it as a
commit you reverted.  Did you leave e8099177 simply because attempting
to revert it fails (if you don't first revert other dm-mpath.c commits)?

(btw, Bart Van Assche also has issues with commit e8099177 due to hangs
during cable pull testing of mpath devices -- Bart: curious to know if
your cable pull tests pass if you just revert bcccff93).

Mike