From cea51128f079d4f05f2220b7425aef507718fa6c Mon Sep 17 00:00:00 2001 From: Tomohiro Kusumi Date: Wed, 10 Jul 2019 01:31:46 +0900 Subject: [PATCH] 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 Reviewed by: Sebastien Roy Signed-off-by: Tomohiro Kusumi Closes #8450 Closes #8833 Closes #8878 --- lib/libzfs/libzfs_mount.c | 6 +- tests/runfiles/linux.run | 3 +- .../functional/cli_root/zfs_mount/Makefile.am | 1 + .../cli_root/zfs_mount/zfs_mount_test_race.sh | 116 ++++++++++++++++++ 4 files changed, 123 insertions(+), 3 deletions(-) create mode 100755 tests/zfs-tests/tests/functional/cli_root/zfs_mount/zfs_mount_test_race.sh diff --git a/lib/libzfs/libzfs_mount.c b/lib/libzfs/libzfs_mount.c index 649c232aa3e5..d62801cfdaca 100644 --- a/lib/libzfs/libzfs_mount.c +++ b/lib/libzfs/libzfs_mount.c @@ -1302,12 +1302,14 @@ mountpoint_cmp(const void *arga, const void *argb) } /* - * Return true if path2 is a child of path1. + * Return true if path2 is a child of path1 or path2 equals path1 or + * path1 is "/" (path2 is always a child of "/"). */ static boolean_t libzfs_path_contains(const char *path1, const char *path2) { - return (strstr(path2, path1) == path2 && path2[strlen(path1)] == '/'); + return (strcmp(path1, path2) == 0 || strcmp(path1, "/") == 0 || + (strstr(path2, path1) == path2 && path2[strlen(path1)] == '/')); } /* diff --git a/tests/runfiles/linux.run b/tests/runfiles/linux.run index 1e2a719b2d0a..9d133632102e 100644 --- a/tests/runfiles/linux.run +++ b/tests/runfiles/linux.run @@ -182,7 +182,8 @@ tests = ['zfs_mount_001_pos', 'zfs_mount_002_pos', 'zfs_mount_003_pos', 'zfs_mount_007_pos', 'zfs_mount_008_pos', 'zfs_mount_009_neg', 'zfs_mount_010_neg', 'zfs_mount_011_neg', 'zfs_mount_012_neg', 'zfs_mount_all_001_pos', 'zfs_mount_encrypted', 'zfs_mount_remount', - 'zfs_multi_mount', 'zfs_mount_all_fail', 'zfs_mount_all_mountpoints'] + 'zfs_multi_mount', 'zfs_mount_all_fail', 'zfs_mount_all_mountpoints', + 'zfs_mount_test_race'] tags = ['functional', 'cli_root', 'zfs_mount'] [tests/functional/cli_root/zfs_program] diff --git a/tests/zfs-tests/tests/functional/cli_root/zfs_mount/Makefile.am b/tests/zfs-tests/tests/functional/cli_root/zfs_mount/Makefile.am index b2de98934b74..c208a1c378dc 100644 --- a/tests/zfs-tests/tests/functional/cli_root/zfs_mount/Makefile.am +++ b/tests/zfs-tests/tests/functional/cli_root/zfs_mount/Makefile.am @@ -19,6 +19,7 @@ dist_pkgdata_SCRIPTS = \ zfs_mount_all_mountpoints.ksh \ zfs_mount_encrypted.ksh \ zfs_mount_remount.ksh \ + zfs_mount_test_race.sh \ zfs_multi_mount.ksh dist_pkgdata_DATA = \ diff --git a/tests/zfs-tests/tests/functional/cli_root/zfs_mount/zfs_mount_test_race.sh b/tests/zfs-tests/tests/functional/cli_root/zfs_mount/zfs_mount_test_race.sh new file mode 100755 index 000000000000..404770b2727f --- /dev/null +++ b/tests/zfs-tests/tests/functional/cli_root/zfs_mount/zfs_mount_test_race.sh @@ -0,0 +1,116 @@ +#!/bin/ksh + +# +# This file and its contents are supplied under the terms of the +# Common Development and Distribution License ("CDDL"), version 1.0. +# You may only use this file in accordance with the terms of version +# 1.0 of the CDDL. +# +# A full copy of the text of the CDDL should have accompanied this +# source. A copy of the CDDL is also available via the Internet at +# http://www.illumos.org/license/CDDL. +# + +# +# Copyright (c) 2019 by Tomohiro Kusumi. All rights reserved. +# + +. $STF_SUITE/include/libtest.shlib +. $STF_SUITE/tests/functional/cli_root/zfs_mount/zfs_mount.cfg + +# +# DESCRIPTION: +# Verify parallel mount ordering is consistent. +# +# There was a bug in initial thread dispatching algorithm which put threads +# under race condition which resulted in undefined mount order. The purpose +# of this test is to verify `zfs unmount -a` succeeds (not `zfs mount -a` +# succeeds, it always does) after `zfs mount -a`, which could fail if threads +# race. See github.com/zfsonlinux/zfs/issues/{8450,8833,8878} for details. +# +# STRATEGY: +# 1. Create pools and filesystems. +# 2. Set same mount point for >1 datasets. +# 3. Unmount all datasets. +# 4. Mount all datasets. +# 5. Unmount all datasets (verify this succeeds). +# + +verify_runnable "both" + +TMPDIR=${TMPDIR:-$TEST_BASE_DIR} +MNTPT=$TMPDIR/zfs_mount_test_race_mntpt +DISK1="$TMPDIR/zfs_mount_test_race_disk1" +DISK2="$TMPDIR/zfs_mount_test_race_disk2" + +TESTPOOL1=zfs_mount_test_race_tp1 +TESTPOOL2=zfs_mount_test_race_tp2 + +export __ZFS_POOL_RESTRICT="$TESTPOOL1 $TESTPOOL2" +log_must zfs $unmountall +unset __ZFS_POOL_RESTRICT + +function cleanup +{ + zpool destroy $TESTPOOL1 + zpool destroy $TESTPOOL2 + rm -rf $MNTPT + rm -rf /$TESTPOOL1 + rm -rf /$TESTPOOL2 + rm -f $DISK1 + rm -f $DISK2 + export __ZFS_POOL_RESTRICT="$TESTPOOL1 $TESTPOOL2" + log_must zfs $mountall + unset __ZFS_POOL_RESTRICT +} +log_onexit cleanup + +log_note "Verify parallel mount ordering is consistent" + +log_must truncate -s $MINVDEVSIZE $DISK1 +log_must truncate -s $MINVDEVSIZE $DISK2 + +log_must zpool create -f $TESTPOOL1 $DISK1 +log_must zpool create -f $TESTPOOL2 $DISK2 + +log_must zfs create $TESTPOOL1/$TESTFS1 +log_must zfs create $TESTPOOL2/$TESTFS2 + +log_must zfs set mountpoint=none $TESTPOOL1 +log_must zfs set mountpoint=$MNTPT $TESTPOOL1/$TESTFS1 + +# Note that unmount can fail (due to race condition on `zfs mount -a`) with or +# without `canmount=off`. The race has nothing to do with canmount property, +# but turn it off for convenience of mount layout used in this test case. +log_must zfs set canmount=off $TESTPOOL2 +log_must zfs set mountpoint=$MNTPT $TESTPOOL2 + +# At this point, layout of datasets in two pools will look like below. +# Previously, on next `zfs mount -a`, pthreads assigned to TESTFS1 and TESTFS2 +# could race, and TESTFS2 usually (actually always) won in ZoL. Note that the +# problem is how two or more threads could initially be assigned to the same +# top level directory, not this specific layout. This layout is just an example +# that can reproduce race, and is also the layout reported in #8833. +# +# NAME MOUNTED MOUNTPOINT +# ---------------------------------------------- +# /$TESTPOOL1 no none +# /$TESTPOOL1/$TESTFS1 yes $MNTPT +# /$TESTPOOL2 no $MNTPT +# /$TESTPOOL2/$TESTFS2 yes $MNTPT/$TESTFS2 + +# Apparently two datasets must be mounted. +log_must ismounted $TESTPOOL1/$TESTFS1 +log_must ismounted $TESTPOOL2/$TESTFS2 +# This unmount always succeeds, because potential race hasn't happened yet. +log_must zfs unmount -a +# This mount always succeeds, whether threads are under race condition or not. +log_must zfs mount -a + +# Verify datasets are mounted (TESTFS2 fails if the race broke mount order). +log_must ismounted $TESTPOOL1/$TESTFS1 +log_must ismounted $TESTPOOL2/$TESTFS2 +# Verify unmount succeeds (fails if the race broke mount order). +log_must zfs unmount -a + +log_pass "Verify parallel mount ordering is consistent passed"