On Tue, Jan 16, 2024 at 03:45:29PM +0000, Glass Su wrote: > > > > On Jan 16, 2024, at 23:32, Brian Foster <[email protected]> wrote: > > > > On Tue, Jan 16, 2024 at 10:10:53PM +0800, Su Yue wrote: > >> > >> On Tue 16 Jan 2024 at 08:47, Brian Foster <[email protected]> wrote: > >> > >>> On Mon, Jan 15, 2024 at 10:21:25AM +0800, Su Yue wrote: > >>>> 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(). > >>>> > >>> > >>> Heh, seems reasonable to me.. > >>> > >>>> Signed-off-by: Su Yue <[email protected]> > >>>> --- > >>>> fs/bcachefs/fs-ioctl.c | 11 +++++------ > >>>> 1 file changed, 5 insertions(+), 6 deletions(-) > >>>> > >>>> diff --git a/fs/bcachefs/fs-ioctl.c b/fs/bcachefs/fs-ioctl.c > >>>> index e0a19a73c8e1..1346861ed944 100644 > >>>> --- a/fs/bcachefs/fs-ioctl.c > >>>> +++ b/fs/bcachefs/fs-ioctl.c > >>>> @@ -337,11 +337,12 @@ static long > >>>> __bch2_ioctl_subvolume_create(struct bch_fs *c, struct file *filp, > >>>> 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, > >>>> @@ -425,8 +426,6 @@ static long __bch2_ioctl_subvolume_create(struct > >>>> bch_fs *c, struct file *filp, > >>>> goto retry; > >>>> } > >>>> err1: > >>>> - up_read(&c->vfs_sb->s_umount); > >>>> - > >>> > >>> Can we get rid of this label now that it serves no purpose? > >>> > >> I think we can't. Two gotos are still there(under the retry label) > >> unless change them to return > >> > > > > Sorry.. yes, that's what I mean. Usually we can just return directly > > rather than goto for cases where there is no necessary clean up on exit. > > > Of course… It’s just about personal flavor and code style. > Actually I don’t like neither of jumping between labels or returning in loop.
I've got no real opinion on the matter (except that I can't wait for Rust so we don't have to be bothered), so applying this :)
