---
- branch: netbsd-8
  date: Sun May  6 09:20:43 UTC 2018
  files:
  - new: 1.172.6.4
    old: 1.172.6.3
    path: src/sys/kern/uipc_mbuf.c
    pathrev: src/sys/kern/uipc_mbuf.c@1.172.6.4
    type: modified
  id: 20180506T092043Z.9e26941feb50a286c83dc038802db15729aaf733
  log: "Pull up following revision(s) (requested by maxv in ticket #802):\n\n\tsys/kern/uipc_mbuf.c:
    revision 1.211 (via patch)\n\nModify m_defrag, so that it never frees the first
    mbuf of the chain. While\nhere use the given 'flags' argument, and not M_DONTWAIT.\n\nWe
    have a problem with several drivers: they poll an mbuf chain from their\nqueues
    and call m_defrag on them, but m_defrag could update the mbuf\npointer, so the
    mbuf in the queue is no longer valid. It is not easy to\nfix each driver, because
    doing pop+push will reorder the queue, and we\ndon't really want that to happen.\n\nThis
    problem was independently spotted by me, Kengo, Masanobu, and other\npeople too
    it seems (perhaps PR/53218).\nNow m_defrag leaves the first mbuf in place, and
    compresses the chain\nonly starting from the second mbuf in the chain.\n\nIt is
    important not to compress the first mbuf with hacks, because the\nstorage of this
    first mbuf may be shared with other mbufs.\n"
  module: src
  subject: 'CVS commit: [netbsd-8] src/sys/kern'
  unixtime: '1525598443'
  user: martin