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4d9380e0da
Often some test cases like btrfs/161 trigger lockdep splats that complain about possible unsafe lock scenario due to the fact that during mount, when reading the chunk tree we end up calling blkdev_get_by_path() while holding a read lock on a leaf of the chunk tree. That produces a lockdep splat like the following: [ 3653.683975] ====================================================== [ 3653.685148] WARNING: possible circular locking dependency detected [ 3653.686301] 5.15.0-rc7-btrfs-next-103 #1 Not tainted [ 3653.687239] ------------------------------------------------------ [ 3653.688400] mount/447465 is trying to acquire lock: [ 3653.689320] ffff8c6b0c76e528 (&disk->open_mutex){+.+.}-{3:3}, at: blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.691054] but task is already holding lock: [ 3653.692155] ffff8c6b0a9f39e0 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x110 [btrfs] [ 3653.693978] which lock already depends on the new lock. [ 3653.695510] the existing dependency chain (in reverse order) is: [ 3653.696915] -> #3 (btrfs-chunk-00){++++}-{3:3}: [ 3653.698053] down_read_nested+0x4b/0x140 [ 3653.698893] __btrfs_tree_read_lock+0x24/0x110 [btrfs] [ 3653.699988] btrfs_read_lock_root_node+0x31/0x40 [btrfs] [ 3653.701205] btrfs_search_slot+0x537/0xc00 [btrfs] [ 3653.702234] btrfs_insert_empty_items+0x32/0x70 [btrfs] [ 3653.703332] btrfs_init_new_device+0x563/0x15b0 [btrfs] [ 3653.704439] btrfs_ioctl+0x2110/0x3530 [btrfs] [ 3653.705405] __x64_sys_ioctl+0x83/0xb0 [ 3653.706215] do_syscall_64+0x3b/0xc0 [ 3653.706990] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 3653.708040] -> #2 (sb_internal#2){.+.+}-{0:0}: [ 3653.708994] lock_release+0x13d/0x4a0 [ 3653.709533] up_write+0x18/0x160 [ 3653.710017] btrfs_sync_file+0x3f3/0x5b0 [btrfs] [ 3653.710699] __loop_update_dio+0xbd/0x170 [loop] [ 3653.711360] lo_ioctl+0x3b1/0x8a0 [loop] [ 3653.711929] block_ioctl+0x48/0x50 [ 3653.712442] __x64_sys_ioctl+0x83/0xb0 [ 3653.712991] do_syscall_64+0x3b/0xc0 [ 3653.713519] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 3653.714233] -> #1 (&lo->lo_mutex){+.+.}-{3:3}: [ 3653.715026] __mutex_lock+0x92/0x900 [ 3653.715648] lo_open+0x28/0x60 [loop] [ 3653.716275] blkdev_get_whole+0x28/0x90 [ 3653.716867] blkdev_get_by_dev.part.0+0x142/0x320 [ 3653.717537] blkdev_open+0x5e/0xa0 [ 3653.718043] do_dentry_open+0x163/0x390 [ 3653.718604] path_openat+0x3f0/0xa80 [ 3653.719128] do_filp_open+0xa9/0x150 [ 3653.719652] do_sys_openat2+0x97/0x160 [ 3653.720197] __x64_sys_openat+0x54/0x90 [ 3653.720766] do_syscall_64+0x3b/0xc0 [ 3653.721285] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 3653.721986] -> #0 (&disk->open_mutex){+.+.}-{3:3}: [ 3653.722775] __lock_acquire+0x130e/0x2210 [ 3653.723348] lock_acquire+0xd7/0x310 [ 3653.723867] __mutex_lock+0x92/0x900 [ 3653.724394] blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.725041] blkdev_get_by_path+0xb8/0xd0 [ 3653.725614] btrfs_get_bdev_and_sb+0x1b/0xb0 [btrfs] [ 3653.726332] open_fs_devices+0xd7/0x2c0 [btrfs] [ 3653.726999] btrfs_read_chunk_tree+0x3ad/0x870 [btrfs] [ 3653.727739] open_ctree+0xb8e/0x17bf [btrfs] [ 3653.728384] btrfs_mount_root.cold+0x12/0xde [btrfs] [ 3653.729130] legacy_get_tree+0x30/0x50 [ 3653.729676] vfs_get_tree+0x28/0xc0 [ 3653.730192] vfs_kern_mount.part.0+0x71/0xb0 [ 3653.730800] btrfs_mount+0x11d/0x3a0 [btrfs] [ 3653.731427] legacy_get_tree+0x30/0x50 [ 3653.731970] vfs_get_tree+0x28/0xc0 [ 3653.732486] path_mount+0x2d4/0xbe0 [ 3653.732997] __x64_sys_mount+0x103/0x140 [ 3653.733560] do_syscall_64+0x3b/0xc0 [ 3653.734080] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 3653.734782] other info that might help us debug this: [ 3653.735784] Chain exists of: &disk->open_mutex --> sb_internal#2 --> btrfs-chunk-00 [ 3653.737123] Possible unsafe locking scenario: [ 3653.737865] CPU0 CPU1 [ 3653.738435] ---- ---- [ 3653.739007] lock(btrfs-chunk-00); [ 3653.739449] lock(sb_internal#2); [ 3653.740193] lock(btrfs-chunk-00); [ 3653.740955] lock(&disk->open_mutex); [ 3653.741431] *** DEADLOCK *** [ 3653.742176] 3 locks held by mount/447465: [ 3653.742739] #0: ffff8c6acf85c0e8 (&type->s_umount_key#44/1){+.+.}-{3:3}, at: alloc_super+0xd5/0x3b0 [ 3653.744114] #1: ffffffffc0b28f70 (uuid_mutex){+.+.}-{3:3}, at: btrfs_read_chunk_tree+0x59/0x870 [btrfs] [ 3653.745563] #2: ffff8c6b0a9f39e0 (btrfs-chunk-00){++++}-{3:3}, at: __btrfs_tree_read_lock+0x24/0x110 [btrfs] [ 3653.747066] stack backtrace: [ 3653.747723] CPU: 4 PID: 447465 Comm: mount Not tainted 5.15.0-rc7-btrfs-next-103 #1 [ 3653.748873] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014 [ 3653.750592] Call Trace: [ 3653.750967] dump_stack_lvl+0x57/0x72 [ 3653.751526] check_noncircular+0xf3/0x110 [ 3653.752136] ? stack_trace_save+0x4b/0x70 [ 3653.752748] __lock_acquire+0x130e/0x2210 [ 3653.753356] lock_acquire+0xd7/0x310 [ 3653.753898] ? blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.754596] ? lock_is_held_type+0xe8/0x140 [ 3653.755125] ? blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.755729] ? blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.756338] __mutex_lock+0x92/0x900 [ 3653.756794] ? blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.757400] ? do_raw_spin_unlock+0x4b/0xa0 [ 3653.757930] ? _raw_spin_unlock+0x29/0x40 [ 3653.758437] ? bd_prepare_to_claim+0x129/0x150 [ 3653.758999] ? trace_module_get+0x2b/0xd0 [ 3653.759508] ? try_module_get.part.0+0x50/0x80 [ 3653.760072] blkdev_get_by_dev.part.0+0xe7/0x320 [ 3653.760661] ? devcgroup_check_permission+0xc1/0x1f0 [ 3653.761288] blkdev_get_by_path+0xb8/0xd0 [ 3653.761797] btrfs_get_bdev_and_sb+0x1b/0xb0 [btrfs] [ 3653.762454] open_fs_devices+0xd7/0x2c0 [btrfs] [ 3653.763055] ? clone_fs_devices+0x8f/0x170 [btrfs] [ 3653.763689] btrfs_read_chunk_tree+0x3ad/0x870 [btrfs] [ 3653.764370] ? kvm_sched_clock_read+0x14/0x40 [ 3653.764922] open_ctree+0xb8e/0x17bf [btrfs] [ 3653.765493] ? super_setup_bdi_name+0x79/0xd0 [ 3653.766043] btrfs_mount_root.cold+0x12/0xde [btrfs] [ 3653.766780] ? rcu_read_lock_sched_held+0x3f/0x80 [ 3653.767488] ? kfree+0x1f2/0x3c0 [ 3653.767979] legacy_get_tree+0x30/0x50 [ 3653.768548] vfs_get_tree+0x28/0xc0 [ 3653.769076] vfs_kern_mount.part.0+0x71/0xb0 [ 3653.769718] btrfs_mount+0x11d/0x3a0 [btrfs] [ 3653.770381] ? rcu_read_lock_sched_held+0x3f/0x80 [ 3653.771086] ? kfree+0x1f2/0x3c0 [ 3653.771574] legacy_get_tree+0x30/0x50 [ 3653.772136] vfs_get_tree+0x28/0xc0 [ 3653.772673] path_mount+0x2d4/0xbe0 [ 3653.773201] __x64_sys_mount+0x103/0x140 [ 3653.773793] do_syscall_64+0x3b/0xc0 [ 3653.774333] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 3653.775094] RIP: 0033:0x7f648bc45aaa This happens because through btrfs_read_chunk_tree(), which is called only during mount, ends up acquiring the mutex open_mutex of a block device while holding a read lock on a leaf of the chunk tree while other paths need to acquire other locks before locking extent buffers of the chunk tree. Since at mount time when we call btrfs_read_chunk_tree() we know that we don't have other tasks running in parallel and modifying the chunk tree, we can simply skip locking of chunk tree extent buffers. So do that and move the assertion that checks the fs is not yet mounted to the top block of btrfs_read_chunk_tree(), with a comment before doing it. Signed-off-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: David Sterba <dsterba@suse.com> |
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Linux kernel
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