bcachefs: grab s_umount only if snapshotting

In the Linux kernel, the following vulnerability has been resolved:

bcachefs: grab s_umount only if snapshotting

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 ***

[ 3437.469

---truncated---