WARNING in __writeback_inodes_sb_nr

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: bbf5c979 Linux 5.9
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1664b377900000
kernel config: https://syzkaller.appspot.com/x/.config?x=3d8333c88fe898d7
dashboard link: https://syzkaller.appspot.com/bug?extid=b8ff83b095e45f39e27e
compiler: gcc (GCC) 10.1.0-syz 20200507
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=13127ef0500000

Bisection is inconclusive: the issue happens on the oldest tested release.

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=14878558500000
final oops: https://syzkaller.appspot.com/x/report.txt?x=16878558500000
console output: https://syzkaller.appspot.com/x/log.txt?x=12878558500000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+b8ff83...@syzkaller.appspotmail.com

BTRFS info (device loop0): disk space caching is enabled
BTRFS info (device loop0): has skinny extents
BTRFS info (device loop0): enabling ssd optimizations
BTRFS info (device loop0): checking UUID tree
------------[ cut here ]------------
WARNING: CPU: 0 PID: 7118 at fs/fs-writeback.c:2469 __writeback_inodes_sb_nr+0x229/0x280 fs/fs-writeback.c:2469
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 7118 Comm: syz-executor.0 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x198/0x1fd lib/dump_stack.c:118
panic+0x382/0x7fb kernel/panic.c:231
__warn.cold+0x20/0x4b kernel/panic.c:600
report_bug+0x1bd/0x210 lib/bug.c:198
handle_bug+0x38/0x90 arch/x86/kernel/traps.c:234
exc_invalid_op+0x14/0x40 arch/x86/kernel/traps.c:254
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:536
RIP: 0010:__writeback_inodes_sb_nr+0x229/0x280 fs/fs-writeback.c:2469
Code: 48 8b 84 24 c0 00 00 00 65 48 2b 04 25 28 00 00 00 75 38 48 81 c4 c8 00 00 00 5b 5d 41 5c 41 5d 41 5e 41 5f c3 e8 f7 75 a7 ff <0f> 0b e9 69 ff ff ff 4c 89 f7 e8 a8 4e e8 ff e9 ea fe ff ff 4c 89
RSP: 0018:ffffc900064076e0 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 1ffff92000c80edd RCX: ffffffff81cec880
RDX: ffff888099db2200 RSI: ffffffff81cec919 RDI: 0000000000000007
RBP: ffff8880994ec000 R08: 0000000000000000 R09: ffff8880994ec077
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffffc90006407708 R14: 0000000000006400 R15: ffff8880994ec158
btrfs_start_delalloc_flush fs/btrfs/transaction.c:1970 [inline]
btrfs_commit_transaction+0x8ea/0x2830 fs/btrfs/transaction.c:2150
btrfs_sync_file+0x821/0xd80 fs/btrfs/file.c:2279
vfs_fsync_range+0x13a/0x220 fs/sync.c:200
generic_write_sync include/linux/fs.h:2747 [inline]
btrfs_file_write_iter+0x1101/0x14a9 fs/btrfs/file.c:2049
call_write_iter include/linux/fs.h:1882 [inline]
do_iter_readv_writev+0x532/0x7b0 fs/read_write.c:721
do_iter_write+0x188/0x670 fs/read_write.c:1026
vfs_writev+0x1aa/0x2e0 fs/read_write.c:1099
do_pwritev fs/read_write.c:1196 [inline]
__do_sys_pwritev fs/read_write.c:1243 [inline]
__se_sys_pwritev fs/read_write.c:1238 [inline]
__x64_sys_pwritev+0x231/0x310 fs/read_write.c:1238
do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45de59
Code: 0d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 db b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f098f185c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000128
RAX: ffffffffffffffda RBX: 0000000000026400 RCX: 000000000045de59
RDX: 0000000000000001 RSI: 00000000200014c0 RDI: 0000000000000003
RBP: 000000000118bf70 R08: 0000000000000020 R09: 0000000000000000
R10: 0000000000000002 R11: 0000000000000246 R12: 000000000118bf2c
R13: 00007ffe7dd36ddf R14: 00007f098f1869c0 R15: 000000000118bf2c
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[Eric Biggers]

+linux-btrfs, for btrfs calling writeback_inodes_sb() from
btrfs_start_delalloc_flush() without holding super_block::s_umount.

[syzbot]

Auto-closing this bug as obsolete.
No recent activity, existing reproducers are no longer triggering the issue.