[PATCH 0/1] printk: Yield console_sem to blocking waiters

[Aruna Ramakrishna <aruna.ramakrishna@oracle.com>]

Problem context:

After a reboot, NetworkManager failed to bring up interfaces because
systemd was stalled, due to a flood of printk warnings. Systemd was
blocked waiting on the console lock for over 122 seconds:

  __schedule+0x224/0x65b
  schedule+0x5f/0xdf
  schedule_timeout+0x110/0x145
  __down_common+0x14d/0x18c
  ? srso_alias_return_thunk+0x5/0x7
  down+0x43/0x54
  console_lock+0x16/0x3d
  show_cons_active+0x3d/0x1b6
  dev_attr_show+0x16/0x50
  sysfs_kf_seq_show+0x9c/0x124
  seq_read_iter+0x12b/0x4e3
  new_sync_read+0x140/0x1db
  vfs_read+0x106/0x199
  ksys_read+0x67/0xe8
  do_syscall_64+0x35/0x87
  entry_SYSCALL_64_after_hwframe+0x6e/0x0

The loglevel was set to 7, due to which there were a flood of messages
to the console, which in turn starved systemd due to the chain of direct
printk handoffs. Direct printk handoffs transfer printing ownership
between printk callers without releasing console_sem. A continuous
chain of these handoffs can therefore leave blocking console_lock()
callers waiting on the semaphore.

Proposed solution:

If there is a blocked console_sem waiter, then:
1. Prevent new printk-to-printk handoffs.
2. Stop the current owner from indefinitely flushing all records until
its done.

This unblocks console_sem waiters who are not doing printk.

Aruna Ramakrishna (1):
  printk: Yield console_sem to blocking waiters

 kernel/printk/printk.c | 39 ++++++++++++++++++++++++++++++++-------
 1 file changed, 32 insertions(+), 7 deletions(-)

-- 
2.43.7