Messages in this thread | | | Date | Fri, 22 Apr 2022 11:21:37 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH printk v4 14/15] printk: extend console_lock for proper kthread support |
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On Thu 2022-04-21 23:28:49, 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 flag CON_THD_BLOCKED to provide this > synchronization. > > console_lock() is modified so that it must acquire the mutex > of each console in order to set the CON_THD_BLOCKED flag. > Console printing threads will acquire their mutex while > printing a record. If CON_THD_BLOCKED was set, the thread will > go back to sleep instead of printing. > > The reason for the CON_THD_BLOCKED flag 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. > > 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->flags |= CON_THD_BLOCKED; > 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->flags & CON_THD_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.
I have finally understood why console_lock_single_hold() solved the problem with con->flags. We should describe it in the commit message as well. Something like:
@con->flags must be updated WRITE_ONCE() under console_lock when the related kthread is running. The kthread printers read the flags only under con->mutex. They have to see the CON_THD_BLOCKED flag when the value might not be consistent.
Sigh, I agree that this approach is error prone and kind of ugly. The approach with console_lock_single_hold() was not ideal either. I still have to think about it.
Anyway, the approach with READ_ONCE()/WRITE_ONCE() looks good enough for now.
> 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 CON_THD_BLOCKED flag > implicitly covers the suspended console case. > > Signed-off-by: John Ogness <john.ogness@linutronix.de>
I do not see any further problem with this patch. So, with the updated commit message and comment above the macros:
Reviewed-by: Petr Mladek <pmladek@suse.com>
Best Regards, Petr
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