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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/10/4 19:33, Johannes Thumshirn 写道:
> 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

My bad, I forgot that the kern_path() can sleep, thus it can not be
utilized inside RCU context.

Instead I should dump the rcu string into a local buffer instead.

And of course I should enable the sleep in atomic for my aarch64 VM too.

Sorry for the RCU related regression again and again.
Qu

> [  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