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 1E6DCC4332F for ; Tue, 26 Apr 2022 13:16:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347563AbiDZNTV (ORCPT ); Tue, 26 Apr 2022 09:19:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33546 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1350900AbiDZNTR (ORCPT ); Tue, 26 Apr 2022 09:19:17 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7F12692333 for ; Tue, 26 Apr 2022 06:16:05 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 3BBD0210F4; Tue, 26 Apr 2022 13:16:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1650978964; 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=8x+8XqjyVG0D0MRWyPaIYxWjcYcFy9j7hzjAUNZpX+s=; b=OTg6l/4bHKqFaIAO+w8tO4m+Tv0pDaxTMkW2VVQ/4EwTxtdqW56ghiEUPlrrKLBF56INzz 6VQZOUG20PsXFgYUyf7iT2NnKWZV/b8+KlUXebEU+X96zo16L/SYulX43VqIY/NSBcCe9H JD2MpYwyqBRsXt3cNOcrWADvqrXGQiw= 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 E217F2C141; Tue, 26 Apr 2022 13:16:03 +0000 (UTC) Date: Tue, 26 Apr 2022 15:16:03 +0200 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Greg Kroah-Hartman 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Best Regards, Petr