bcachefs: grab s_umount only if snapshotting

Summary

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---