From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH printk v3 14/15] printk: extend console_lock for proper kthread support
Date: Mon, 25 Apr 2022 17:18:47 +0200 [thread overview]
Message-ID: <Yma71x2p10d6yOLU@alley> (raw)
In-Reply-To: <877d7gu7yg.fsf@jogness.linutronix.de>
On Fri 2022-04-22 23:31:11, John Ogness wrote:
> On 2022-04-22, Petr Mladek <pmladek@suse.com> wrote:
> > Another problem is that the ordering is not stable. The console
> > might come and go.
>
> The console list is protected by @console_sem, so it wouldn't be an
> actual problem. The real issue is that lockdep would not like it. A new
> lockdep class would need to be setup for each register_console().
Yeah. I did not mention it explicitely but I meant it as a problem
with lockdep.
> >> Anyway, I will first look into the nested locking solution. That
> >> seems more promising to me and it would go a long way to simplify the
> >> locking hierarchy.
> >
> > Please, do not spend too much time on this. The solution must be
> > simple in principle. If it gets complicated than it will likely
> > be worse than the current code.
>
> Sure. The goal is to simplify. The only complexity will be doing in a
> way that allow lockdep to understand it.
I am not sure how to distinguish intentional and non-intentional
ordering change.
> > Alternative solution would be to reduce the number of variables
> > affected by the race. I mean:
> >
> > + replace CON_THB_BLOCKED flag with con->blocked to avoid
> > the needed of READ_ONCE()/WRITE_ONCE().
> >
> > + check con->blocked right after taking con->lock in
> > printk_kthread_func() so that all the other accesses are
> > safe.
>
> Honestly, I would prefer this to what v4 is doing. The only reason
> CON_THD_BLOCKED is a flag is to save space. But we are only talking
> about a few bytes being saved. There aren't that many consoles.
>
> It would be a very simple change. Literally just replacing the 3 lines
> that set/clear CON_THD_BLOCKED and replacing/reordering the 2 lines that
> check the flag. Then all the READ_ONCE/WRITE_ONCE to @flags could be
> removed.
I agree that it sounds like the easiest solution for now. If you
prepare v5 with this change then I push it into linux-next instead
of v4.
Well, I think that we need to make con->lock safe to use in the long
term. The above workaround in printk_kthread_func() is good enough
for now because this is the only location where con->lock is taken without
console_sem. But I am sure that we/people will want to do more
console-specific operations without console_sem in the future.
IMHO, the only sane approach is to follow the proposed rules:
+ console_lock() will synchronize both global and per-console
stuff.
+ con->lock will synchronize per-console stuff.
+ con->lock could not be taken alone when the big console_lock()
is taken.
I currently know only about two solutions:
1. The nested locking. console_lock() will take console_sem
and all con->lock's and will keep them locked.
It is rather trivial in principle. The problem is lockdep
and possible ABBA deadlocks caused by unstable ordering.
2. Create the wrappers around con->lock that will check
whether console_sem is taken (con->locked flag).
It will require additional per-console waitqueue. But all
the magic will be hidden in the wrappers.
I personally prefer 2nd approach for the long term solution. It might
look more complicated but it will not break lockdep.
Best Regards,
Petr
next prev parent reply other threads:[~2022-04-25 15:18 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-19 23:46 [PATCH printk v3 00/15] printk/for-next John Ogness
2022-04-19 23:46 ` [PATCH printk v3 01/15] printk: rename cpulock functions John Ogness
2022-04-19 23:46 ` [PATCH printk v3 02/15] printk: cpu sync always disable interrupts John Ogness
2022-04-19 23:46 ` [PATCH printk v3 03/15] printk: add missing memory barrier to wake_up_klogd() John Ogness
2022-04-20 12:34 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 04/15] printk: wake up all waiters John Ogness
2022-04-20 12:36 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 05/15] printk: wake waiters for safe and NMI contexts John Ogness
2022-04-20 13:55 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 06/15] printk: get caller_id/timestamp after migration disable John Ogness
2022-04-19 23:46 ` [PATCH printk v3 07/15] printk: call boot_delay_msec() in printk_delay() John Ogness
2022-04-19 23:46 ` [PATCH printk v3 08/15] printk: add con_printk() macro for console details John Ogness
2022-04-20 14:01 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 09/15] printk: refactor and rework printing logic John Ogness
2022-04-20 14:55 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 10/15] printk: move buffer definitions into console_emit_next_record() caller John Ogness
2022-04-19 23:46 ` [PATCH printk v3 11/15] printk: add pr_flush() John Ogness
2022-04-20 15:10 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 12/15] printk: add functions to prefer direct printing John Ogness
2022-04-19 23:46 ` [PATCH printk v3 13/15] printk: add kthread console printers John Ogness
2022-04-20 17:53 ` Petr Mladek
2022-04-20 20:02 ` John Ogness
2022-04-21 14:25 ` Petr Mladek
2022-04-19 23:46 ` [PATCH printk v3 14/15] printk: extend console_lock for proper kthread support John Ogness
2022-04-20 2:13 ` kernel test robot
2022-04-20 13:32 ` John Ogness
2022-04-20 4:04 ` kernel test robot
2022-04-21 12:41 ` Petr Mladek
2022-04-21 14:30 ` John Ogness
2022-04-22 13:03 ` Petr Mladek
2022-04-22 14:14 ` John Ogness
2022-04-22 15:15 ` Petr Mladek
2022-04-22 21:25 ` John Ogness
2022-04-25 15:18 ` Petr Mladek [this message]
2022-04-25 19:10 ` John Ogness
2022-04-19 23:46 ` [PATCH printk v3 15/15] printk: remove @console_locked John Ogness
2022-04-21 12:46 ` Petr Mladek
2022-04-21 14:40 ` [PATCH printk v3 00/15] printk/for-next Petr Mladek
2022-04-21 15:02 ` John Ogness
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