From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3C299C433EF for ; Wed, 27 Apr 2022 07:38:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1358950AbiD0HlY (ORCPT ); Wed, 27 Apr 2022 03:41:24 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56906 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1353952AbiD0HlW (ORCPT ); Wed, 27 Apr 2022 03:41:22 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C66BDD17EA for ; Wed, 27 Apr 2022 00:38:11 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 6E11D1F746; Wed, 27 Apr 2022 07:38:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1651045090; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=57pWPR14Ne9tA5nJvzDyYoG3eEBDpwAnTWbaFiNJcLo=; b=YAf5B2D8R9QJpD27vbCviOY55n3pEtVEmNPLHxrve0FRpwGqd7K15Ri5nJsQwpeGeO6XFK Lq0zcDL307LHA1QI+q7nGdlqlFpjxDWu1uI3JbgqPByuhwf6dNqjgm2l57/wWNbnljeOxC F7AhfzghcWugNbVEM5IVPxWNNuMl5D0= Received: from suse.cz (unknown [10.100.224.162]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id 1DF6F2C141; Wed, 27 Apr 2022 07:38:09 +0000 (UTC) Date: Wed, 27 Apr 2022 09:38:06 +0200 From: Petr Mladek To: Marek Szyprowski Cc: John Ogness , Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Greg Kroah-Hartman , linux-amlogic@lists.infradead.org Subject: Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking Message-ID: References: <20220421212250.565456-1-john.ogness@linutronix.de> <20220421212250.565456-15-john.ogness@linutronix.de> <878rrs6ft7.fsf@jogness.linutronix.de> <2a82eae7-a256-f70c-fd82-4e510750906e@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2a82eae7-a256-f70c-fd82-4e510750906e@samsung.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2022-04-27 09:08:33, Marek Szyprowski wrote: > 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 > >> Nice, it it better than v4. I am going to push this for linux-next. > >> > >> Reviewed-by: Petr Mladek > > 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. Thanks a lot for the report! Just by chance, do you have the log from the dead-locked boot stored in userspace and can you share it? I mean the log stored in /var/log/dmesg or journaltctl. In the worst case, it might help to see log from the boot with the reverted patch. I would help us to see the ordering of various console-related operations on your system. And regarding the console. Is it the graphics console (ttyX) or a serial one (ttyS) or yet another one? Best Regards, Petr