6.7-stable review patch. If anyone has any objections, please let me know.
------------------
From: Su Yue glass.su@suse.com
commit 2acc59dd88d27ad69b66ded80df16c042b04eeec upstream.
When I was testing mongodb over bcachefs with compression, there is a lockdep warning when snapshotting mongodb data volume.
$ cat test.sh prog=bcachefs
$prog subvolume create /mnt/data $prog subvolume create /mnt/data/snapshots
while true;do $prog subvolume snapshot /mnt/data /mnt/data/snapshots/$(date +%s) sleep 1s done
$ cat /etc/mongodb.conf systemLog: destination: file logAppend: true path: /mnt/data/mongod.log
storage: dbPath: /mnt/data/
lockdep reports: [ 3437.452330] ====================================================== [ 3437.452750] WARNING: possible circular locking dependency detected [ 3437.453168] 6.7.0-rc7-custom+ #85 Tainted: G E [ 3437.453562] ------------------------------------------------------ [ 3437.453981] bcachefs/35533 is trying to acquire lock: [ 3437.454325] ffffa0a02b2b1418 (sb_writers#10){.+.+}-{0:0}, at: filename_create+0x62/0x190 [ 3437.454875] but task is already holding lock: [ 3437.455268] ffffa0a02b2b10e0 (&type->s_umount_key#48){.+.+}-{3:3}, at: bch2_fs_file_ioctl+0x232/0xc90 [bcachefs] [ 3437.456009] which lock already depends on the new lock.
[ 3437.456553] the existing dependency chain (in reverse order) is: [ 3437.457054] -> #3 (&type->s_umount_key#48){.+.+}-{3:3}: [ 3437.457507] down_read+0x3e/0x170 [ 3437.457772] bch2_fs_file_ioctl+0x232/0xc90 [bcachefs] [ 3437.458206] __x64_sys_ioctl+0x93/0xd0 [ 3437.458498] do_syscall_64+0x42/0xf0 [ 3437.458779] entry_SYSCALL_64_after_hwframe+0x6e/0x76 [ 3437.459155] -> #2 (&c->snapshot_create_lock){++++}-{3:3}: [ 3437.459615] down_read+0x3e/0x170 [ 3437.459878] bch2_truncate+0x82/0x110 [bcachefs] [ 3437.460276] bchfs_truncate+0x254/0x3c0 [bcachefs] [ 3437.460686] notify_change+0x1f1/0x4a0 [ 3437.461283] do_truncate+0x7f/0xd0 [ 3437.461555] path_openat+0xa57/0xce0 [ 3437.461836] do_filp_open+0xb4/0x160 [ 3437.462116] do_sys_openat2+0x91/0xc0 [ 3437.462402] __x64_sys_openat+0x53/0xa0 [ 3437.462701] do_syscall_64+0x42/0xf0 [ 3437.462982] entry_SYSCALL_64_after_hwframe+0x6e/0x76 [ 3437.463359] -> #1 (&sb->s_type->i_mutex_key#15){+.+.}-{3:3}: [ 3437.463843] down_write+0x3b/0xc0 [ 3437.464223] bch2_write_iter+0x5b/0xcc0 [bcachefs] [ 3437.464493] vfs_write+0x21b/0x4c0 [ 3437.464653] ksys_write+0x69/0xf0 [ 3437.464839] do_syscall_64+0x42/0xf0 [ 3437.465009] entry_SYSCALL_64_after_hwframe+0x6e/0x76 [ 3437.465231] -> #0 (sb_writers#10){.+.+}-{0:0}: [ 3437.465471] __lock_acquire+0x1455/0x21b0 [ 3437.465656] lock_acquire+0xc6/0x2b0 [ 3437.465822] mnt_want_write+0x46/0x1a0 [ 3437.465996] filename_create+0x62/0x190 [ 3437.466175] user_path_create+0x2d/0x50 [ 3437.466352] bch2_fs_file_ioctl+0x2ec/0xc90 [bcachefs] [ 3437.466617] __x64_sys_ioctl+0x93/0xd0 [ 3437.466791] do_syscall_64+0x42/0xf0 [ 3437.466957] entry_SYSCALL_64_after_hwframe+0x6e/0x76 [ 3437.467180] other info that might help us debug this:
[ 3437.469670] 2 locks held by bcachefs/35533: other info that might help us debug this:
[ 3437.467507] Chain exists of: sb_writers#10 --> &c->snapshot_create_lock --> &type->s_umount_key#48
[ 3437.467979] Possible unsafe locking scenario:
[ 3437.468223] CPU0 CPU1 [ 3437.468405] ---- ---- [ 3437.468585] rlock(&type->s_umount_key#48); [ 3437.468758] lock(&c->snapshot_create_lock); [ 3437.469030] lock(&type->s_umount_key#48); [ 3437.469291] rlock(sb_writers#10); [ 3437.469434] *** DEADLOCK ***
[ 3437.469670] 2 locks held by bcachefs/35533: [ 3437.469838] #0: ffffa0a02ce00a88 (&c->snapshot_create_lock){++++}-{3:3}, at: bch2_fs_file_ioctl+0x1e3/0xc90 [bcachefs] [ 3437.470294] #1: ffffa0a02b2b10e0 (&type->s_umount_key#48){.+.+}-{3:3}, at: bch2_fs_file_ioctl+0x232/0xc90 [bcachefs] [ 3437.470744] stack backtrace: [ 3437.470922] CPU: 7 PID: 35533 Comm: bcachefs Kdump: loaded Tainted: G E 6.7.0-rc7-custom+ #85 [ 3437.471313] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014 [ 3437.471694] Call Trace: [ 3437.471795] <TASK> [ 3437.471884] dump_stack_lvl+0x57/0x90 [ 3437.472035] check_noncircular+0x132/0x150 [ 3437.472202] __lock_acquire+0x1455/0x21b0 [ 3437.472369] lock_acquire+0xc6/0x2b0 [ 3437.472518] ? filename_create+0x62/0x190 [ 3437.472683] ? lock_is_held_type+0x97/0x110 [ 3437.472856] mnt_want_write+0x46/0x1a0 [ 3437.473025] ? filename_create+0x62/0x190 [ 3437.473204] filename_create+0x62/0x190 [ 3437.473380] user_path_create+0x2d/0x50 [ 3437.473555] bch2_fs_file_ioctl+0x2ec/0xc90 [bcachefs] [ 3437.473819] ? lock_acquire+0xc6/0x2b0 [ 3437.474002] ? __fget_files+0x2a/0x190 [ 3437.474195] ? __fget_files+0xbc/0x190 [ 3437.474380] ? lock_release+0xc5/0x270 [ 3437.474567] ? __x64_sys_ioctl+0x93/0xd0 [ 3437.474764] ? __pfx_bch2_fs_file_ioctl+0x10/0x10 [bcachefs] [ 3437.475090] __x64_sys_ioctl+0x93/0xd0 [ 3437.475277] do_syscall_64+0x42/0xf0 [ 3437.475454] entry_SYSCALL_64_after_hwframe+0x6e/0x76 [ 3437.475691] RIP: 0033:0x7f2743c313af ======================================================
In __bch2_ioctl_subvolume_create(), we grab s_umount unconditionally and unlock it at the end of the function. There is a comment "why do we need this lock?" about the lock coming from commit 42d237320e98 ("bcachefs: Snapshot creation, deletion") The reason is that __bch2_ioctl_subvolume_create() calls sync_inodes_sb() which enforce locked s_umount to writeback all dirty nodes before doing snapshot works.
Fix it by read locking s_umount for snapshotting only and unlocking s_umount after sync_inodes_sb().
Signed-off-by: Su Yue glass.su@suse.com Signed-off-by: Kent Overstreet kent.overstreet@linux.dev Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/bcachefs/fs-ioctl.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-)
--- a/fs/bcachefs/fs-ioctl.c +++ b/fs/bcachefs/fs-ioctl.c @@ -345,11 +345,12 @@ static long __bch2_ioctl_subvolume_creat if (arg.flags & BCH_SUBVOL_SNAPSHOT_RO) create_flags |= BCH_CREATE_SNAPSHOT_RO;
- /* why do we need this lock? */ - down_read(&c->vfs_sb->s_umount); - - if (arg.flags & BCH_SUBVOL_SNAPSHOT_CREATE) + if (arg.flags & BCH_SUBVOL_SNAPSHOT_CREATE) { + /* sync_inodes_sb enforce s_umount is locked */ + down_read(&c->vfs_sb->s_umount); sync_inodes_sb(c->vfs_sb); + up_read(&c->vfs_sb->s_umount); + } retry: if (arg.src_ptr) { error = user_path_at(arg.dirfd, @@ -433,8 +434,6 @@ err2: goto retry; } err1: - up_read(&c->vfs_sb->s_umount); - return error; }