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 X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 04BB3C4361A for ; Fri, 4 Dec 2020 16:11:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AFF342223E for ; Fri, 4 Dec 2020 16:11:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730741AbgLDQKy (ORCPT ); Fri, 4 Dec 2020 11:10:54 -0500 Received: from mx2.suse.de ([195.135.220.15]:35462 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727096AbgLDQKy (ORCPT ); Fri, 4 Dec 2020 11:10:54 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1607098207; 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=D33SdhwZNJi2uBaYJpFAFfeHS3fk9aEn8alrEz4biwo=; b=R4nG2RhjBXbm0SfRECtr8SQfRq+9lcVM56MVjR5KL1t3L6IlXzTgIxsu3BwXuEFoZC4bvm edF09KND73+4y/drqmjOe6NkMKrRUVAhHPDTlfEVK7JJ8tXebIAjiKrUDGqxMJKL+NFT8p ZDHlLQfGNfa9u1rsEknSMnZGXFfQRiQ= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 81BDCAC9A; Fri, 4 Dec 2020 16:10:07 +0000 (UTC) Date: Fri, 4 Dec 2020 17:10:07 +0100 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: recursion handling: Re: [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog_lock Message-ID: References: <20201201205341.3871-1-john.ogness@linutronix.de> <20201201205341.3871-4-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201201205341.3871-4-john.ogness@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2020-12-01 21:59:41, John Ogness wrote: > Since the ringbuffer is lockless, there is no need for it to be > protected by @logbuf_lock. Remove @logbuf_lock. > > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -1847,6 +1811,65 @@ static void call_console_drivers(const char *ext_text, size_t ext_len, > } > } The recursion-related code needs some explanation or we should do it another way. I spent quite some time on it and I am still not sure that I understand it. Let me describe how I understand it. > +#ifdef CONFIG_PRINTK_NMI > +#define NUM_RECURSION_CTX 2 > +#else > +#define NUM_RECURSION_CTX 1 > +#endif OK, the number of context is limited because interrupts are disabled inside print_enter()/printk_exit(). It is basically the same reason why we have only two printk_safe buffers (NNI + other contexts). What is the exact reason to disable interrupts around the entire vprintk_store(), please? It should get documented. One reason is the use of per-cpu variables. Alternative solution would be to store printk_context into task_struct. Well, I am not sure if "current" task is available during early boot. But it might solve problems with per-cpu variables that are not working during early boot. That said, I am not sure if it is worth it. > + > +struct printk_recursion { > + char count[NUM_RECURSION_CTX]; > +}; > > +static DEFINE_PER_CPU(struct printk_recursion, percpu_printk_recursion); > +static char printk_recursion_count[NUM_RECURSION_CTX]; This is pretty confusing. The array is hidden in a struct when per-cpu variables are used. And a naked array is used for early boot. Is the structure really needed? What about? static DEFINE_PER_CPU(char [PRINTK_CTX_NUM], printk_count); static char printk_count_early[NUM_RECURSION_CTX]; > + > +static char *get_printk_count(void) > +{ > + struct printk_recursion *rec; > + char *count; > + > + if (!printk_percpu_data_ready()) { > + count = &printk_recursion_count[0]; I see why you avoided per-cpu variables for early boot. I am just curious how printk_context variable works these days. It is used by any printk(), including early code, see vprintk_func(). > + } else { > + rec = this_cpu_ptr(&percpu_printk_recursion); > + > + count = &rec->count[0]; > + } > + > +#ifdef CONFIG_PRINTK_NMI > + if (in_nmi()) > + count++; > +#endif This is extremely confusing. It is far from obvious that the pointer and not the value is incremented. If we really need this to avoid per-cpu variables during early boot then a more clear implementation would be: char *get_printk_counter_by_ctx() { int ctx = 0; if (in_nmi) ctx = 1; if (!printk_percpu_data_ready()) return &printk_count_early[ctx]; return this_cpu_ptr(printk_count[ctx]); } > + > + return count; > +} > + > +static bool printk_enter(unsigned long *flags) > +{ > + char *count; > + > + local_irq_save(*flags); > + count = get_printk_count(); > + /* Only 1 level of recursion allowed. */ We should allow at least some level of recursion. Otherwise, we would not see warnings printed from vsprintf code. > + if (*count > 1) { > + local_irq_restore(*flags); > + return false; > + } > + (*count)++; > + > + return true; > +} This should be unified with printk_context, printk_nmi_enter(), printk_nmi_exit(). It does not make sense to have two separate printk context counters. Or is there any plan to remove printk_safe and printk_context? BTW: I prefer to use the bitmask approach. It allows to check the normal context by a single operation (no bit is set). There is no need to go through all counters. Note that we need at least one more context for gdb. Best Regards, Petr