On (07/11/17 11:31), Sergey Senozhatsky wrote:
[..]
> (replying to both Petr and Daniel)
>
> interesting direction, gents.
>
> and this is what I thought about over the weekend; it's very sketchy and
> I didn't spend too much time on it. (I'm on a sick leave now, sorry).
>
> it's quite close to what you guys have mentioned above.
>
> a) keep console_sem only to protect console drivers list modification
> b) add a semaphore/mutex to struct console
> c) move global console_seq/etc to struct console
> e) use a single kthread for printing, but do console_unlock() multi passes,
> printing unseen logbuf messages on per-console basis
>
>
> so console_lock()/console_unlock() will simply protect console drivers
> list from concurrent manipulation; it will not prevent us from printing.
> now, there are places where console_lock() serves a special purpose - it
> makes sure that no new lines are printed to the console while we scroll
> it/flip it/etc. IOW while we do "some things" to a particular console.
> the problem here, is that this also blocks printing to all of the registered
> console drivers, not just the one we are touching now. therefore, what I was
> thinking about is to disable/enable that particular console in all of the
> places where we really want to stop printing to this console for a bit.
>
> IOW, something like
>
>
>
> console_lock()
> : down(console_sem);
>
> console_disable(con)
> : lock(con->lock);
> : con->flags &= ~CON_ENABLED;
> : unlock(con->lock)
>
> console_unlock()
> : for_each_console(con)
> : while (con->console_seq != log_next_seq) {
> : msg_print_text();
> : con->console_seq++;
> :
> : call_console_drivers()
> : : if (con->flags & CON_ENABLED)
> : : con->write()
> : }
> : up(console_sem);
>
>
> // do "some things" to this console. it's disabled, so no
> // ->write() callback would be called in the meantime
>
> console_lock()
> : down(console_sem);
>
> console_enable(con)
> : lock(con->lock);
> : con->flags |= CON_ENABLED;
> : unlock(con->lock)
>
>
> // so now we enabled that console again. it's ->console_seq is
> // probably behind the rest of consoles, so console_unlock()
> // will ->write() all the unseen message to this console.
>
> console_unlock()
> : for_each_console(con)
> : while (con->console_seq != log_next_seq) {
> : msg_print_text();
> : con->console_seq++;
> :
> : call_console_drivers()
> : : if (con->flags & CON_ENABLED)
> : : con->write()
> : }
> : up(console_sem);
>
ok, obviously stupid.
I meant to hold con->lock between console_disable() and console_enable().
so no other CPU can unregister it, etc. printk->console_unlock(), thus,
can either have a racy con->flags check (no con->lock taken) or try
something like down_trylock(&con->lock): if it fails, continue.
but need to look more.
-ss
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