Messages in this thread | | | Date | Fri, 8 Apr 2022 15:52:37 +0200 | From | Petr Mladek <> | Subject | register_console: was: Re: [PATCH printk v2 11/12] printk: extend console_lock for proper kthread support |
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On Tue 2022-04-05 15:31:34, 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. > > However, 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. Since threaded printers never take > the console lock, they do not have a console lock to handover to > other contexts. > > @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.
BTW: I really like the commit message.
> --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -3230,7 +3335,11 @@ void register_console(struct console *newcon) > if (newcon->flags & CON_EXTENDED) > nr_ext_console_drivers++; > > + if (console_kthreads_blocked)
I think that this is actully always true because it is called under console_lock().
> + newcon->flags |= CON_THD_BLOCKED;
> + > newcon->dropped = 0; > + mutex_init(&newcon->lock); > if (newcon->flags & CON_PRINTBUFFER) { > /* Get a consistent copy of @syslog_seq. */ > mutex_lock(&syslog_lock);
Best Regards, Petr
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