[PATCH AUTOSEL 5.12 49/63] btrfs: release path before starting transaction when cloning inline extent

From: Sasha Levin
Date: Mon May 24 2021 - 10:51:00 EST

From: Filipe Manana <fdmanana@xxxxxxxx>

[ Upstream commit 6416954ca75baed71640bf3828625bf165fb9b5e ]

When cloning an inline extent there are a few cases, such as when we have
an implicit hole at file offset 0, where we start a transaction while
holding a read lock on a leaf. Starting the transaction results in a call
to sb_start_intwrite(), which results in doing a read lock on a percpu
semaphore. Lockdep doesn't like this and complains about it:

[46.580704] ======================================================
[46.580752] WARNING: possible circular locking dependency detected
[46.580799] 5.13.0-rc1 #28 Not tainted
[46.580832] ------------------------------------------------------
[46.580877] cloner/3835 is trying to acquire lock:
[46.580918] c00000001301d638 (sb_internal#2){.+.+}-{0:0}, at: clone_copy_inline_extent+0xe4/0x5a0
[46.581167]
[46.581167] but task is already holding lock:
[46.581217] c000000007fa2550 (btrfs-tree-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x70/0x1d0
[46.581293]
[46.581293] which lock already depends on the new lock.
[46.581293]
[46.581351]
[46.581351] the existing dependency chain (in reverse order) is:
[46.581410]
[46.581410] -> #1 (btrfs-tree-00){++++}-{3:3}:
[46.581464] down_read_nested+0x68/0x200
[46.581536] __btrfs_tree_read_lock+0x70/0x1d0
[46.581577] btrfs_read_lock_root_node+0x88/0x200
[46.581623] btrfs_search_slot+0x298/0xb70
[46.581665] btrfs_set_inode_index+0xfc/0x260
[46.581708] btrfs_new_inode+0x26c/0x950
[46.581749] btrfs_create+0xf4/0x2b0
[46.581782] lookup_open.isra.57+0x55c/0x6a0
[46.581855] path_openat+0x418/0xd20
[46.581888] do_filp_open+0x9c/0x130
[46.581920] do_sys_openat2+0x2ec/0x430
[46.581961] do_sys_open+0x90/0xc0
[46.581993] system_call_exception+0x3d4/0x410
[46.582037] system_call_common+0xec/0x278
[46.582078]
[46.582078] -> #0 (sb_internal#2){.+.+}-{0:0}:
[46.582135] __lock_acquire+0x1e90/0x2c50
[46.582176] lock_acquire+0x2b4/0x5b0
[46.582263] start_transaction+0x3cc/0x950
[46.582308] clone_copy_inline_extent+0xe4/0x5a0
[46.582353] btrfs_clone+0x5fc/0x880
[46.582388] btrfs_clone_files+0xd8/0x1c0
[46.582434] btrfs_remap_file_range+0x3d8/0x590
[46.582481] do_clone_file_range+0x10c/0x270
[46.582558] vfs_clone_file_range+0x1b0/0x310
[46.582605] ioctl_file_clone+0x90/0x130
[46.582651] do_vfs_ioctl+0x874/0x1ac0
[46.582697] sys_ioctl+0x6c/0x120
[46.582733] system_call_exception+0x3d4/0x410
[46.582777] system_call_common+0xec/0x278
[46.582822]
[46.582822] other info that might help us debug this:
[46.582822]
[46.582888] Possible unsafe locking scenario:
[46.582888]
[46.582942] CPU0 CPU1
[46.582984] ---- ----
[46.583028] lock(btrfs-tree-00);
[46.583062] lock(sb_internal#2);
[46.583119] lock(btrfs-tree-00);
[46.583174] lock(sb_internal#2);
[46.583212]
[46.583212] *** DEADLOCK ***
[46.583212]
[46.583266] 6 locks held by cloner/3835:
[46.583299] #0: c00000001301d448 (sb_writers#12){.+.+}-{0:0}, at: ioctl_file_clone+0x90/0x130
[46.583382] #1: c00000000f6d3768 (&sb->s_type->i_mutex_key#15){+.+.}-{3:3}, at: lock_two_nondirectories+0x58/0xc0
[46.583477] #2: c00000000f6d72a8 (&sb->s_type->i_mutex_key#15/4){+.+.}-{3:3}, at: lock_two_nondirectories+0x9c/0xc0
[46.583574] #3: c00000000f6d7138 (&ei->i_mmap_lock){+.+.}-{3:3}, at: btrfs_remap_file_range+0xd0/0x590
[46.583657] #4: c00000000f6d35f8 (&ei->i_mmap_lock/1){+.+.}-{3:3}, at: btrfs_remap_file_range+0xe0/0x590
[46.583743] #5: c000000007fa2550 (btrfs-tree-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x70/0x1d0
[46.583828]
[46.583828] stack backtrace:
[46.583872] CPU: 1 PID: 3835 Comm: cloner Not tainted 5.13.0-rc1 #28
[46.583931] Call Trace:
[46.583955] [c0000000167c7200] [c000000000c1ee78] dump_stack+0xec/0x144 (unreliable)
[46.584052] [c0000000167c7240] [c000000000274058] print_circular_bug.isra.32+0x3a8/0x400
[46.584123] [c0000000167c72e0] [c0000000002741f4] check_noncircular+0x144/0x190
[46.584191] [c0000000167c73b0] [c000000000278fc0] __lock_acquire+0x1e90/0x2c50
[46.584259] [c0000000167c74f0] [c00000000027aa94] lock_acquire+0x2b4/0x5b0
[46.584317] [c0000000167c75e0] [c000000000a0d6cc] start_transaction+0x3cc/0x950
[46.584388] [c0000000167c7690] [c000000000af47a4] clone_copy_inline_extent+0xe4/0x5a0
[46.584457] [c0000000167c77c0] [c000000000af525c] btrfs_clone+0x5fc/0x880
[46.584514] [c0000000167c7990] [c000000000af5698] btrfs_clone_files+0xd8/0x1c0
[46.584583] [c0000000167c7a00] [c000000000af5b58] btrfs_remap_file_range+0x3d8/0x590
[46.584652] [c0000000167c7ae0] [c0000000005d81dc] do_clone_file_range+0x10c/0x270
[46.584722] [c0000000167c7b40] [c0000000005d84f0] vfs_clone_file_range+0x1b0/0x310
[46.584793] [c0000000167c7bb0] [c00000000058bf80] ioctl_file_clone+0x90/0x130
[46.584861] [c0000000167c7c10] [c00000000058c894] do_vfs_ioctl+0x874/0x1ac0
[46.584922] [c0000000167c7d10] [c00000000058db4c] sys_ioctl+0x6c/0x120
[46.584978] [c0000000167c7d60] [c0000000000364a4] system_call_exception+0x3d4/0x410
[46.585046] [c0000000167c7e10] [c00000000000d45c] system_call_common+0xec/0x278
[46.585114] --- interrupt: c00 at 0x7ffff7e22990
[46.585160] NIP: 00007ffff7e22990 LR: 00000001000010ec CTR: 0000000000000000
[46.585224] REGS: c0000000167c7e80 TRAP: 0c00 Not tainted (5.13.0-rc1)
[46.585280] MSR: 800000000280f033 <SF,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE> CR: 28000244 XER: 00000000
[46.585374] IRQMASK: 0
[46.585374] GPR00: 0000000000000036 00007fffffffdec0 00007ffff7f17100 0000000000000004
[46.585374] GPR04: 000000008020940d 00007fffffffdf40 0000000000000000 0000000000000000
[46.585374] GPR08: 0000000000000004 0000000000000000 0000000000000000 0000000000000000
[46.585374] GPR12: 0000000000000000 00007ffff7ffa940 0000000000000000 0000000000000000
[46.585374] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[46.585374] GPR20: 0000000000000000 000000009123683e 00007fffffffdf40 0000000000000000
[46.585374] GPR24: 0000000000000000 0000000000000000 0000000000000000 0000000000000004
[46.585374] GPR28: 0000000100030260 0000000100030280 0000000000000003 000000000000005f
[46.585919] NIP [00007ffff7e22990] 0x7ffff7e22990
[46.585964] LR [00000001000010ec] 0x1000010ec
[46.586010] --- interrupt: c00

This should be a false positive, as both locks are acquired in read mode.
Nevertheless, we don't need to hold a leaf locked when we start the
transaction, so just release the leaf (path) before starting it.

Reported-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/linux-btrfs/20210513214404.xks77p566fglzgum@riteshh-domain/
Reviewed-by: Anand Jain <anand.jain@xxxxxxxxxx>
Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
fs/btrfs/reflink.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/fs/btrfs/reflink.c b/fs/btrfs/reflink.c
index 0abbf050580d..53ee17f5e382 100644
--- a/fs/btrfs/reflink.c
+++ b/fs/btrfs/reflink.c
@@ -285,6 +285,11 @@ static int clone_copy_inline_extent(struct inode *dst,
ret = btrfs_inode_set_file_extent_range(BTRFS_I(dst), 0, aligned_end);
out:
if (!ret && !trans) {
+ /*
+ * Release path before starting a new transaction so we don't
+ * hold locks that would confuse lockdep.
+ */
+ btrfs_release_path(path);
/*
* No transaction here means we copied the inline extent into a
* page of the destination inode.
--
2.30.2