Re: [PATCH] btrfs: fix the wrong rcu_str_deref() usage

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 02.10.24 23:35, Qu Wenruo wrote:
> For rcu_str_deref(), it should be called with rcu read lock hold.
> But inside the function is_same_device(), the rcu string is accessed
> without holding the rcu read lock, causing rcu warnings.
> 
> Fix it by holding the rcu read lock during the path resolution of the
> existing device.
> 
> This will be folded into the offending patch "btrfs: avoid unnecessary
> device path update for the same device"
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>   fs/btrfs/volumes.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 35c4d151b5b0..3867d3c18be5 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -807,8 +807,10 @@ static bool is_same_device(struct btrfs_device *device, const char *new_path)
>   	if (!device->name)
>   		goto out;
>   
> +	rcu_read_lock();
>   	old_path = rcu_str_deref(device->name);
>   	ret = kern_path(old_path, LOOKUP_FOLLOW, &old);
> +	rcu_read_unlock();
>   	if (ret)
>   		goto out;
>   	ret = kern_path(new_path, LOOKUP_FOLLOW, &new);


With that patch applied I get the following in btrfs/006 with Qemu 
emulated NVMe drives:
[  150.421039] run fstests btrfs/006 at 2024-10-04 09:50:53
[  151.322158] BTRFS: device fsid e9489dbd-0346-4c23-9b8b-9a0221e235f8 
devid 1 transid 8 /dev/nvme0n1 (259:1) scanned by mount (29282)
[  151.322779] BTRFS info (device nvme0n1): first mount of filesystem 
e9489dbd-0346-4c23-9b8b-9a0221e235f8
[  151.322789] BTRFS info (device nvme0n1): using crc32c (crc32c-intel) 
checksum algorithm
[  151.322794] BTRFS info (device nvme0n1): using free-space-tree
[  152.831300] BTRFS: device fsid 05148ba4-7ee1-480f-af62-7aa0b80b35f2 
devid 1 transid 6 /dev/nvme1n1 (259:0) scanned by mkfs.btrfs (29450)
[  152.831712] BTRFS: device fsid 05148ba4-7ee1-480f-af62-7aa0b80b35f2 
devid 2 transid 6 /dev/nvme2n1 (259:3) scanned by mkfs.btrfs (29450)
[  152.832292] BTRFS: device fsid 05148ba4-7ee1-480f-af62-7aa0b80b35f2 
devid 3 transid 6 /dev/nvme3n1 (259:2) scanned by mkfs.btrfs (29450)
[  152.832747] BTRFS: device fsid 05148ba4-7ee1-480f-af62-7aa0b80b35f2 
devid 4 transid 6 /dev/nvme4n1 (259:4) scanned by mkfs.btrfs (29450)
[  152.833189] BTRFS: device fsid 05148ba4-7ee1-480f-af62-7aa0b80b35f2 
devid 5 transid 6 /dev/nvme5n1 (259:5) scanned by mkfs.btrfs (29450)
[  152.872989] BUG: sleeping function called from invalid context at 
include/linux/sched/mm.h:337
[  152.872995] in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 
29459, name: (udev-worker)
[  152.872997] preempt_count: 0, expected: 0
[  152.872999] RCU nest depth: 1, expected: 0
[  152.873001] 3 locks held by (udev-worker)/29459:
[  152.873003]  #0: ffffffff8253e968 (uuid_mutex){....}-{3:3}, at: 
btrfs_control_ioctl+0xcb/0x1a0
[  152.873016]  #1: ffff8881105c18d8 
(&fs_devs->device_list_mutex){....}-{3:3}, at: 
device_list_add.constprop.0+0x126/0x670
[  152.873027]  #2: ffffffff824cdb60 (rcu_read_lock){....}-{1:2}, at: 
device_list_add.constprop.0+0x193/0x670
[  152.873036] CPU: 0 UID: 0 PID: 29459 Comm: (udev-worker) Tainted: G 
      W          6.11.0-rc7+ #800
[  152.873040] Tainted: [W]=WARN
[  152.873042] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[  152.873044] Call Trace:
[  152.873047]  <TASK>
[  152.873050]  dump_stack_lvl+0x4b/0x70
[  152.873056]  __might_resched.cold+0xbf/0xcf
[  152.873061]  kmem_cache_alloc_noprof+0x1c5/0x2e0
[  152.873067]  ? getname_kernel+0x29/0x150
[  152.873073]  getname_kernel+0x29/0x150
[  152.873077]  kern_path+0x17/0x50
[  152.873081]  device_list_add.constprop.0+0x1c1/0x670
[  152.873085]  ? device_list_add.constprop.0+0x193/0x670
[  152.873089]  ? device_list_add.constprop.0+0x193/0x670
[  152.873092]  ? btrfs_scan_one_device+0x1bf/0x410
[  152.873101]  btrfs_scan_one_device+0x239/0x410
[  152.873112]  btrfs_control_ioctl+0xdb/0x1a0
[  152.873119]  __x64_sys_ioctl+0x96/0xc0
[  152.873127]  do_syscall_64+0x54/0x110
[  152.873132]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[  152.873139] RIP: 0033:0x7fa8daf99f9b
[  152.873143] Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 
00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 
05 <89> c2 3d 00 f0 ff ff 77 1c 48 8b 44 24 18 64 48 2b 04 25 28 00 00
[  152.873146] RSP: 002b:00007ffd34cd5cf0 EFLAGS: 00000246 ORIG_RAX: 
0000000000000010
[  152.873150] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 
00007fa8daf99f9b
[  152.873152] RDX: 00007ffd34cd5d70 RSI: 0000000090009427 RDI: 
0000000000000005
[  152.873154] RBP: 00007ffd34cd5d70 R08: 0000000000000000 R09: 
00007ffd34cd6cd0
[  152.873156] R10: 0000000000000000 R11: 0000000000000246 R12: 
00005639a2ce2b60
[  152.873158] R13: 0000000000000000 R14: 0000000000000000 R15: 
0000000000000005
[  152.873166]  </TASK>
[  153.117462] BTRFS info (device nvme1n1): first mount of filesystem 
05148ba4-7ee1-480f-af62-7aa0b80b35f2
[  153.117475] BTRFS info (device nvme1n1): using crc32c (crc32c-intel) 
checksum algorithm
[  153.117480] BTRFS info (device nvme1n1): using free-space-tree
[  153.120689] BTRFS info (device nvme1n1): checking UUID tree
[  153.632137] BTRFS info (device nvme0n1): last unmount of filesystem 
e9489dbd-0346-4c23-9b8b-9a0221e235f8
[  153.715256] BTRFS info (device nvme1n1): last unmount of filesystem 
05148ba4-7ee1-480f-af62-7aa0b80b35f2
[  153.818188] BTRFS info (device nvme1n1): first mount of filesystem 
05148ba4-7ee1-480f-af62-7aa0b80b35f2
[  153.818203] BTRFS info (device nvme1n1): using crc32c (crc32c-intel) 
checksum algorithm
[  153.818209] BTRFS info (device nvme1n1): using free-space-tree
[  153.884093] BTRFS info (device nvme1n1): last unmount of filesystem 
05148ba4-7ee1-480f-af62-7aa0b80b35f2