zfs-patches/0005-Fix-deadlock-between-zfs-umount-snapentry_expire.patch

   1 From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
   2 From: Rohan Puri <rohan.puri15@gmail.com>
   3 Date: Sat, 28 Jul 2018 18:32:12 +0530
   4 Subject: [PATCH] Fix deadlock between zfs umount & snapentry_expire
   5
   6 zfs umount -> zfsctl_destroy() takes the zfs_snapshot_lock as a
   7 writer and calls zfsctl_snapshot_unmount_cancel(), which waits
   8 for snapentry_expire() if present (when snap is automounted).
   9 This snapentry_expire() itself then waits for zfs_snapshot_lock
  10 as a reader, resulting in a deadlock.
  11
  12 The fix is to only hold the zfs_snapshot_lock over the tree
  13 lookup and removal.  After a successful lookup the lock can
  14 be dropped and zfs_snapentry_t will remain valid until the
  15 reference taken by the lookup is released.
  16
  17 Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
  18 Signed-off-by: Rohan Puri <rohan.puri15@gmail.com>
  19 Closes #7751
  20 Closes #7752
  21
  22 (Cherry-picked from fd7265c646f40e364396af5014bbb83e809e124a)
  23 Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
  24 ---
  25  module/zfs/zfs_ctldir.c | 11 +++++------
  26  1 file changed, 5 insertions(+), 6 deletions(-)
  27
  28 diff --git a/module/zfs/zfs_ctldir.c b/module/zfs/zfs_ctldir.c
  29 index 3b5fb196..14af55c4 100644
  30 --- a/module/zfs/zfs_ctldir.c
  31 +++ b/module/zfs/zfs_ctldir.c
  32 @@ -358,8 +358,6 @@ snapentry_expire(void *data)
  33  static void
  34  zfsctl_snapshot_unmount_cancel(zfs_snapentry_t *se)
  35  {
  36 -       ASSERT(RW_LOCK_HELD(&zfs_snapshot_lock));
  37 -
  38         if (taskq_cancel_id(system_delay_taskq, se->se_taskqid) == 0) {
  39                 se->se_taskqid = TASKQID_INVALID;
  40                 zfsctl_snapshot_rele(se);
  41 @@ -570,13 +568,14 @@ zfsctl_destroy(zfsvfs_t *zfsvfs)
  42                 uint64_t objsetid = dmu_objset_id(zfsvfs->z_os);
  43
  44                 rw_enter(&zfs_snapshot_lock, RW_WRITER);
  45 -               if ((se = zfsctl_snapshot_find_by_objsetid(spa, objsetid))
  46 -                   != NULL) {
  47 -                       zfsctl_snapshot_unmount_cancel(se);
  48 +               se = zfsctl_snapshot_find_by_objsetid(spa, objsetid);
  49 +               if (se != NULL)
  50                         zfsctl_snapshot_remove(se);
  51 +               rw_exit(&zfs_snapshot_lock);
  52 +               if (se != NULL) {
  53 +                       zfsctl_snapshot_unmount_cancel(se);
  54                         zfsctl_snapshot_rele(se);
  55                 }
  56 -               rw_exit(&zfs_snapshot_lock);
  57         } else if (zfsvfs->z_ctldir) {
  58                 iput(zfsvfs->z_ctldir);
  59                 zfsvfs->z_ctldir = NULL;