[Bug 237397] 'zfs mount -a' mounts filesystems in incorrect order

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Mon Jul 29 08:24:14 UTC 2019


https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237397

--- Comment #8 from commit-hook at freebsd.org ---
A commit references this bug:

Author: bapt
Date: Mon Jul 29 08:23:15 UTC 2019
New revision: 350402
URL: https://svnweb.freebsd.org/changeset/base/350402

Log:
  MFC r350358:

  Fix a bug introduced with parallel mounting of zfs

  Incorporate a fix from zol:
 
https://github.com/zfsonlinux/zfs/commit/ab5036df1ccbe1b18c1ce6160b5829e8039d94ce

  commit log from upstream:
   Fix race in parallel mount's thread dispatching algorithm

  Strategy of parallel mount is as follows.

  1) Initial thread dispatching is to select sets of mount points that
   don't have dependencies on other sets, hence threads can/should run
   lock-less and shouldn't race with other threads for other sets. Each
   thread dispatched corresponds to top level directory which may or may
   not have datasets to be mounted on sub directories.

  2) Subsequent recursive thread dispatching for each thread from 1)
   is to mount datasets for each set of mount points. The mount points
   within each set have dependencies (i.e. child directories), so child
   directories are processed only after parent directory completes.

  The problem is that the initial thread dispatching in
  zfs_foreach_mountpoint() can be multi-threaded when it needs to be
  single-threaded, and this puts threads under race condition. This race
  appeared as mount/unmount issues on ZoL for ZoL having different
  timing regarding mount(2) execution due to fork(2)/exec(2) of mount(8).
  `zfs unmount -a` which expects proper mount order can't unmount if the
  mounts were reordered by the race condition.

  There are currently two known patterns of input list `handles` in
  `zfs_foreach_mountpoint(..,handles,..)` which cause the race condition.

  1) #8833 case where input is `/a /a /a/b` after sorting.
   The problem is that libzfs_path_contains() can't correctly handle an
   input list with two same top level directories.
   There is a race between two POSIX threads A and B,
    * ThreadA for "/a" for test1 and "/a/b"
    * ThreadB for "/a" for test0/a
   and in case of #8833, ThreadA won the race. Two threads were created
   because "/a" wasn't considered as `"/a" contains "/a"`.

  2) #8450 case where input is `/ /var/data /var/data/test` after sorting.
   The problem is that libzfs_path_contains() can't correctly handle an
   input list containing "/".
   There is a race between two POSIX threads A and B,
    * ThreadA for "/" and "/var/data/test"
    * ThreadB for "/var/data"
   and in case of #8450, ThreadA won the race. Two threads were created
   because "/var/data" wasn't considered as `"/" contains "/var/data"`.
   In other words, if there is (at least one) "/" in the input list,
   the initial thread dispatching must be single-threaded since every
   directory is a child of "/", meaning they all directly or indirectly
   depend on "/".

  In both cases, the first non_descendant_idx() call fails to correctly
  determine "path1-contains-path2", and as a result the initial thread
  dispatching creates another thread when it needs to be single-threaded.
  Fix a conditional in libzfs_path_contains() to consider above two.

  Reviewed-by: Brian Behlendorf <behlendorf1 at llnl.gov>
  Reviewed by: Sebastien Roy <sebastien.roy at delphix.com>
  Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro at gmail.com>

  PR:  237517, 237397, 239243
  Submitted by: Matthew D. Fuller <fullermd at over-yonder.net> (by email)

Changes:
_U  stable/11/
  stable/11/cddl/contrib/opensolaris/lib/libzfs/common/libzfs_mount.c

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