This is because we have system-locales ebuild, which relies on the same version of glibc. So I have to update system-locales and glibc at the same time, because of the dependency, it builds glibc for the target.

I am wondering can we relax the dependency of system-locales to upgrade glibc first, after that, we upgrade system-locales.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/9ee4db3b19814cf73a642e882b471513b4a04386

commit 9ee4db3b19814cf73a642e882b471513b4a04386
Author: Yunlian Jiang <yunlian@google.com>
Date: Thu Dec 15 00:54:00 2016

Change the depenency of system-locales.

The system-locales requires exactly the same version as glibc. However,
we could not upgrade glibc and system-locales in the same time. This is
because the glibc is built by the sdk bot, but the system-locales is
built for the target. If we upgrade these two package at the same time,
in the commit queue buildbot, the buildbot will try to build glibc for
target and it will fail. So glibc and system-locales can fall out of
sync during an upgrade.

BUG= chromium:674336 
TEST=cbuildbot.

Change-Id: Iacfde8063914472b540661f1cd29faabc4905047
Reviewed-on: https://chromium-review.googlesource.com/419696
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[rename] https://crrev.com/9ee4db3b19814cf73a642e882b471513b4a04386/chromeos-base/system-locales/system-locales-2.19-r1.ebuild

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/f8dea149c230bd061d3664c78c1924a94cfa914e

commit f8dea149c230bd061d3664c78c1924a94cfa914e
Author: Yunlian Jiang <yunlian@google.com>
Date: Fri Jan 06 18:25:38 2017

update system-locales version.

glibc update finishes, we need to update system-locales
accordingly.

BUG= chromium:674336 
TEST=emerge-falco system-locales works.

Change-Id: I182e5fbb9e9d1caba7c4c3584934ab462a6e8144
Reviewed-on: https://chromium-review.googlesource.com/425918
Commit-Ready: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

[rename] https://crrev.com/f8dea149c230bd061d3664c78c1924a94cfa914e/chromeos-base/system-locales/system-locales-2.23.ebuild

Closing. Please reopen it if its not fixed. Thanks!