Problem:
=======

The current locking mechanism in netconsole is unsafe and suboptimal due
to the following issues:

1) Lock Release and Reacquisition Mid-Loop:

In netconsole_netdev_event(), the target_list_lock is released and
reacquired within a loop, potentially causing collisions and cleaning up
targets that are being enabled.

	int netconsole_netdev_event()
	{
	...
		spin_lock_irqsave(&target_list_lock, flags);
		list_for_each_entry(nt, &target_list, list) {
			spin_unlock_irqrestore(&target_list_lock, flags);
			__netpoll_cleanup(&nt->np);
			spin_lock_irqsave(&target_list_lock, flags);
		}
		spin_lock_irqsave(&target_list_lock, flags);
	...
	}

2) Non-Atomic Cleanup Operations:

In enabled_store(), the cleanup of structures is not atomic, risking
cleanup of structures that are in the process of being enabled.

	size_t enabled_store()
	{
	...
		spin_lock_irqsave(&target_list_lock, flags);
		nt->enabled = false;
		spin_unlock_irqrestore(&target_list_lock, flags);
		netpoll_cleanup(&nt->np);
	...
	}


These issues stem from the following limitations in netconsole's locking
design:

1) write_{ext_}msg() functions:

	a) Cannot sleep
	b) Must iterate through targets and send messages to all enabled entries.
	c) List iteration is protected by target_list_lock spinlock.

2) Network event handling in netconsole_netdev_event():

	a) Needs to sleep
	b) Requires iteration over the target list (holding
	   target_list_lock spinlock).
	c) Some events necessitate netpoll struct cleanup, which *needs*
	   to sleep.

The target_list_lock needs to be used by non-sleepable functions while
also protecting operations that may sleep, leading to the current unsafe
design.


Solution:
========

1) Dual Locking Mechanism:
	- Retain current target_list_lock for non-sleepable use cases.
	- Introduce target_cleanup_list_lock (mutex) for sleepable
	  operations.

2) Deferred Cleanup:
	- Implement atomic, deferred cleanup of structures using the new
	  mutex (target_cleanup_list_lock).
	- Avoid the `goto` in the middle of the list_for_each_entry

3) Separate Cleanup List:
	- Create target_cleanup_list for deferred cleanup, protected by
	  target_cleanup_list_lock.
	- This allows cleanup() to sleep without affecting message
	  transmission.
	- When iterating over targets, move devices needing cleanup to
	  target_cleanup_list.
	- Handle cleanup under the target_cleanup_list_lock mutex.

4) Make a clear locking hierarchy

	- The target_cleanup_list_lock takes precedence over target_list_lock.

	- Major Workflow Locking Sequences:
		a) Network Event Affecting Netpoll (netconsole_netdev_event):
			rtnl -> target_cleanup_list_lock -> target_list_lock

		b) Message Writing (write_msg()):
			console_lock -> target_list_lock

		c) Configfs Target Enable/Disable (enabled_store()):
			dynamic_netconsole_mutex -> target_cleanup_list_lock -> target_list_lock


This hierarchy ensures consistent lock acquisition order across
different operations, preventing deadlocks and maintaining proper
synchronization. The target_cleanup_list_lock's higher priority allows
for safe deferred cleanup operations without interfering with regular
message transmission protected by target_list_lock.  Each workflow
follows a specific locking sequence, ensuring that operations like
network event handling, message writing, and target management are
properly synchronized and do not conflict with each other.


Testing:
=======

This patchset contains a basic version of the test I am using to test
netconsole. I hope to include it into NIPA also.

I've also tested with some parallel dynamic removal/add, and network
events, and I haven't seen any issue. Tested with KASAN and LOCKDEP.


Breno Leitao (6):
  net: netconsole: selftests: Create a new netconsole selftest
  net: netpoll: extract core of netpoll_cleanup
  net: netconsole: Correct mismatched return types
  net: netconsole: Standardize variable naming
  net: netconsole: Unify Function Return Paths
  net: netconsole: Defer netpoll cleanup to avoid lock release during
    list traversal

 MAINTAINERS                                   |   1 +
 drivers/net/netconsole.c                      | 173 +++++++++++-------
 include/linux/netpoll.h                       |   1 +
 net/core/netpoll.c                            |  12 +-
 .../net/netconsole/basic_integration_test.sh  | 153 ++++++++++++++++
 5 files changed, 272 insertions(+), 68 deletions(-)
 create mode 100755 tools/testing/selftests/net/netconsole/basic_integration_test.sh

-- 
2.43.0