Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi,

On 26.04.2022 15:16, Petr Mladek wrote:
> On Tue 2022-04-26 14:07:42, Petr Mladek wrote:
>> On Mon 2022-04-25 23:04:28, John Ogness wrote:
>>> Currently threaded console printers synchronize against each
>>> other using console_lock(). However, different console drivers
>>> are unrelated and do not require any synchronization between
>>> each other. Removing the synchronization between the threaded
>>> console printers will allow each console to print at its own
>>> speed.
>>>
>>> But the threaded consoles printers do still need to synchronize
>>> against console_lock() callers. Introduce a per-console mutex
>>> and a new console boolean field @blocked to provide this
>>> synchronization.
>>>
>>> console_lock() is modified so that it must acquire the mutex
>>> of each console in order to set the @blocked field. Console
>>> printing threads will acquire their mutex while printing a
>>> record. If @blocked was set, the thread will go back to sleep
>>> instead of printing.
>>>
>>> The reason for the @blocked boolean field is so that
>>> console_lock() callers do not need to acquire multiple console
>>> mutexes simultaneously, which would introduce unnecessary
>>> complexity due to nested mutex locking. Also, a new field
>>> was chosen instead of adding a new @flags value so that the
>>> blocked status could be checked without concern of reading
>>> inconsistent values due to @flags updates from other contexts.
>>>
>>> Threaded console printers also need to synchronize against
>>> console_trylock() callers. Since console_trylock() may be
>>> called from any context, the per-console mutex cannot be used
>>> for this synchronization. (mutex_trylock() cannot be called
>>> from atomic contexts.) Introduce a global atomic counter to
>>> identify if any threaded printers are active. The threaded
>>> printers will also check the atomic counter to identify if the
>>> console has been locked by another task via console_trylock().
>>>
>>> Note that @console_sem is still used to provide synchronization
>>> between console_lock() and console_trylock() callers.
>>>
>>> A locking overview for console_lock(), console_trylock(), and the
>>> threaded printers is as follows (pseudo code):
>>>
>>> console_lock()
>>> {
>>>          down(&console_sem);
>>>          for_each_console(con) {
>>>                  mutex_lock(&con->lock);
>>>                  con->blocked = true;
>>>                  mutex_unlock(&con->lock);
>>>          }
>>>          /* console_lock acquired */
>>> }
>>>
>>> console_trylock()
>>> {
>>>          if (down_trylock(&console_sem) == 0) {
>>>                  if (atomic_cmpxchg(&console_kthreads_active, 0, -1) == 0) {
>>>                          /* console_lock acquired */
>>>                  }
>>>          }
>>> }
>>>
>>> threaded_printer()
>>> {
>>>          mutex_lock(&con->lock);
>>>          if (!con->blocked) {
>>> 		/* console_lock() callers blocked */
>>>
>>>                  if (atomic_inc_unless_negative(&console_kthreads_active)) {
>>>                          /* console_trylock() callers blocked */
>>>
>>>                          con->write();
>>>
>>>                          atomic_dec(&console_lock_count);
>>>                  }
>>>          }
>>>          mutex_unlock(&con->lock);
>>> }
>>>
>>> The console owner and waiter logic now only applies between contexts
>>> that have taken the console_lock via console_trylock(). Threaded
>>> printers never take the console_lock, so they do not have a
>>> console_lock to handover. Tasks that have used console_lock() will
>>> block the threaded printers using a mutex and if the console_lock
>>> is handed over to an atomic context, it would be unable to unblock
>>> the threaded printers. However, the console_trylock() case is
>>> really the only scenario that is interesting for handovers anyway.
>>>
>>> @panic_console_dropped must change to atomic_t since it is no longer
>>> protected exclusively by the console_lock.
>>>
>>> Since threaded printers remain asleep if they see that the console
>>> is locked, they now must be explicitly woken in __console_unlock().
>>> This means wake_up_klogd() calls following a console_unlock() are
>>> no longer necessary and are removed.
>>>
>>> Also note that threaded printers no longer need to check
>>> @console_suspended. The check for the @blocked field implicitly
>>> covers the suspended console case.
>>>
>>> Signed-off-by: John Ogness <john.ogness@linutronix.de>
>> Nice, it it better than v4. I am going to push this for linux-next.
>>
>> Reviewed-by: Petr Mladek <pmladek@suse.com>
> JFYI, I have just pushed this patch instead of the one
> from v4 into printk/linux.git, branch rework/kthreads.
>
> It means that this branch has been rebased. It will be
> used in the next refresh of linux-next.

This patchset landed in linux next-20220426. In my tests I've found that 
it causes deadlock on all my Amlogic Meson G12B/SM1 based boards: Odroid 
C4/N2 and Khadas VIM3/VIM3l. The deadlock happens when system boots to 
userspace and getty (with automated login) is executed. I even see the 
bash prompt, but then the console is freezed. Reverting this patch 
(e00cc0e1cbf4) on top of linux-next (together with 6b3d71e87892 to make 
revert clean) fixes the issue.


Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland