Describe CVE-2021-47510.
What is CVE-2021-47510?
In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix re-dirty process of tree-log nodes

There is a report of a transaction abort of -EAGAIN with the following
script.

  #!/bin/sh

  for d in sda sdb; do
          mkfs.btrfs -d single -m single -f /dev/\${d}
  done

  mount /dev/sda /mnt/test
  mount /dev/sdb /mnt/scratch

  for dir in test scratch; do
          echo 3 >/proc/sys/vm/drop_caches
          fio --directory=/mnt/\${dir} --name=fio.\${dir} --rw=read --size=50G --bs=64m \
                  --numjobs=$(nproc) --time_based --ramp_time=5 --runtime=480 \
                  --group_reporting |& tee /dev/shm/fio.\${dir}
          echo 3 >/proc/sys/vm/drop_caches
  done

  for d in sda sdb; do
          umount /dev/\${d}
  done

The stack trace is shown in below.

  [3310.967991] BTRFS: error (device sda) in btrfs_commit_transaction:2341: errno=-11 unknown (Error while writing out transaction)
  [3310.968060] BTRFS info (device sda): forced readonly
  [3310.968064] BTRFS warning (device sda): Skipping commit of aborted transaction.
  [3310.968065] ------------[ cut here ]------------
  [3310.968066] BTRFS: Transaction aborted (error -11)
  [3310.968074] WARNING: CPU: 14 PID: 1684 at fs/btrfs/transaction.c:1946 btrfs_commit_transaction.cold+0x209/0x2c8
  [3310.968131] CPU: 14 PID: 1684 Comm: fio Not tainted 5.14.10-300.fc35.x86_64 #1
  [3310.968135] Hardware name: DIAWAY Tartu/Tartu, BIOS V2.01.B10 04/08/2021
  [3310.968137] RIP: 0010:btrfs_commit_transaction.cold+0x209/0x2c8
  [3310.968144] RSP: 0018:ffffb284ce393e10 EFLAGS: 00010282
  [3310.968147] RAX: 0000000000000026 RBX: ffff973f147b0f60 RCX: 0000000000000027
  [3310.968149] RDX: ffff974ecf098a08 RSI: 0000000000000001 RDI: ffff974ecf098a00
  [3310.968150] RBP: ffff973f147b0f08 R08: 0000000000000000 R09: ffffb284ce393c48
  [3310.968151] R10: ffffb284ce393c40 R11: ffffffff84f47468 R12: ffff973f101bfc00
  [3310.968153] R13: ffff971f20cf2000 R14: 00000000fffffff5 R15: ffff973f147b0e58
  [3310.968154] FS:  00007efe65468740(0000) GS:ffff974ecf080000(0000) knlGS:0000000000000000
  [3310.968157] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [3310.968158] CR2: 000055691bcbe260 CR3: 000000105cfa4001 CR4: 0000000000770ee0
  [3310.968160] PKRU: 55555554
  [3310.968161] Call Trace:
  [3310.968167]  ? dput+0xd4/0x300
  [3310.968174]  btrfs_sync_file+0x3f1/0x490
  [3310.968180]  __x64_sys_fsync+0x33/0x60
  [3310.968185]  do_syscall_64+0x3b/0x90
  [3310.968190]  entry_SYSCALL_64_after_hwframe+0x44/0xae
  [3310.968194] RIP: 0033:0x7efe6557329b
  [3310.968200] RSP: 002b:00007ffe0236ebc0 EFLAGS: 00000293 ORIG_RAX: 000000000000004a
  [3310.968203] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007efe6557329b
  [3310.968204] RDX: 0000000000000000 RSI: 00007efe58d77010 RDI: 0000000000000006
  [3310.968205] RBP: 0000000004000000 R08: 0000000000000000 R09: 00007efe58d77010
  [3310.968207] R10: 0000000016cacc0c R11: 0000000000000293 R12: 00007efe5ce95980
  [3310.968208] R13: 0000000000000000 R14: 00007efe6447c790 R15: 0000000c80000000
  [3310.968212] ---[ end trace 1a346f4d3c0d96ba ]---
  [3310.968214] BTRFS: error (device sda) in cleanup_transaction:1946: errno=-11 unknown

The abort occurs because of a write hole while writing out freeing tree
nodes of a tree-log tree. For zoned btrfs, we re-dirty a freed tree
node to ensure btrfs can write the region and does not leave a hole on
write on a zoned device. The current code fails to re-dirty a node
when the tree-log tree's depth is greater or equal to 2. That leads to
a transaction abort with -EAGAIN.

Fix the issue by properly re-dirtying a node on walking up the tree..