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[Chris Friesen <chris.friesen@xxxxxxxxxxxxx>]

Public bug reported:

In git commit a79ecbe Russel Bryant submitted a partial fix for a race
condition when booting an instance as part of a server group with an
"anti-affinity" scheduler policy.

That fix only solves part of the problem, however.  There are a number
of issues remaining:

1) It's possible to hit a similar race condition for server groups with
the "affinity" policy.  Suppose we create a new group and then create
two instances simultaneously.  The scheduler sees an empty group for
each, assigns them to different compute nodes, and the policy is
violated.  We should add a check in _validate_instance_group_policy() to
cover the "affinity" case.

2) It's possible to create two instances simultaneously, have them be
scheduled to conflicting hosts, both of them detect the problem in
_validate_instance_group_policy(), both of them get sent back for
rescheduling, and both of them get assigned to conflicting hosts
*again*, resulting in an error.  In order to fix this I propose that
instead of checking against all other instances in the group, we only
check against instances that were created before the current instance.

** Affects: nova
     Importance: Undecided
         Status: New


** Tags: compute

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https://bugs.launchpad.net/bugs/1423648

Title:
  race conditions with server group scheduler policies

Status in OpenStack Compute (Nova):
  New

Bug description:
  In git commit a79ecbe Russel Bryant submitted a partial fix for a race
  condition when booting an instance as part of a server group with an
  "anti-affinity" scheduler policy.

  That fix only solves part of the problem, however.  There are a number
  of issues remaining:

  1) It's possible to hit a similar race condition for server groups
  with the "affinity" policy.  Suppose we create a new group and then
  create two instances simultaneously.  The scheduler sees an empty
  group for each, assigns them to different compute nodes, and the
  policy is violated.  We should add a check in
  _validate_instance_group_policy() to cover the "affinity" case.

  2) It's possible to create two instances simultaneously, have them be
  scheduled to conflicting hosts, both of them detect the problem in
  _validate_instance_group_policy(), both of them get sent back for
  rescheduling, and both of them get assigned to conflicting hosts
  *again*, resulting in an error.  In order to fix this I propose that
  instead of checking against all other instances in the group, we only
  check against instances that were created before the current instance.

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