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ccd8125e45
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@llnl.gov> Reviewed by: Sebastien Roy <sebastien.roy@delphix.com> Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com> Closes #8450 Closes #8833 Closes #8878 |
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ZFS on Linux is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community.
Official Resources
Installation
Full documentation for installing ZoL on your favorite Linux distribution can be found at our site.
Contribute & Develop
We have a separate document with contribution guidelines.
Release
ZFS on Linux is released under a CDDL license.
For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197
Supported Kernels
- The
METAfile contains the officially recognized supported kernel versions.