Summary:

  Asserting INIT might block kdump if the system is already going to
  start kdump via panic.

Description:

  INIT can interrupt anywhere in panic path, so it can interrupt in
  middle of kdump kicked by panic.  Therefore there is a race if kdump
  is kicked concurrently, via Panic and via INIT.

  INIT could fail to invoke kdump if the system is already going to
  start kdump via panic.  It could not restart kdump from INIT handler
  if some of cpus are already playing dead with INIT masked.  It also
  means that INIT could block kdump's progress if no monarch is entered
  in the INIT rendezvous.

  Panic+INIT is a rare, but possible situation since it can be assumed
  that the kernel or an internal agent decides to panic the unstable
  system while another external agent decides to send an INIT to the
  system at same time.

How to reproduce:

  Assert INIT just after panic, before all other cpus have frozen

Expected results:

  continue kdump invoked by panic, or restart kdump from INIT

Actual results:

  might be hang, crashdump not retrieved

Proposed Fix:

  This patch masks INIT first in panic path to take the initiative on
  kdump, and reuse atomic value kdump_in_progress to make sure there is
  only one initiator of kdump.  All INITs asserted later should be used
  only for freezing all other cpus.

  This mask will be removed soon by rfi in relocate_kernel.S, before jump
  into kdump kernel, after all cpus are frozen and no-op INIT handler is
  registered.  So if INIT was in the interval while it is masked, it will
  pend on the system and will received just after the rfi, and handled by
  the no-op handler.

  If there was a MCA event while psr.mc is 1, in theory the event will
  pend on the system and will received just after the rfi same as above.
  MCA handler is unregistered here at the time, so received MCA will not
  reach to OS_MCA and will result in warmboot by SAL.

  Note that codes in this masked interval are relatively simpler than
  that in MCA/INIT handler which also executed with the mask.  So it can
  be said that probability of error in this interval is supposed not so
  higher than that in MCA/INIT handler.
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Haren Myneni <hbabu@us.ibm.com>
Cc: kexec@lists.infradead.org
Acked-by: Fenghua Yu <fenghua.yu@intel.com>