From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752881AbcLLOMm (ORCPT ); Mon, 12 Dec 2016 09:12:42 -0500 Received: from mail-pg0-f67.google.com ([74.125.83.67]:36007 "EHLO mail-pg0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752228AbcLLOMk (ORCPT ); Mon, 12 Dec 2016 09:12:40 -0500 Date: Mon, 12 Dec 2016 23:12:30 +0900 From: Sergey Senozhatsky To: Petr Mladek Cc: Sergey Senozhatsky , Andrew Morton , Jan Kara , Tejun Heo , Calvin Owens , Thomas Gleixner , Steven Rostedt , Ingo Molnar , Peter Zijlstra , Andy Lutomirski , Linus Torvalds , linux-kernel@vger.kernel.org, Sergey Senozhatsky Subject: Re: [RFC][PATCHv5 3/7] printk: introduce per-cpu safe_print seq buffer Message-ID: <20161212141230.GA2118@tigerII.localdomain> References: <20161201135546.15549-1-sergey.senozhatsky@gmail.com> <20161201135546.15549-4-sergey.senozhatsky@gmail.com> <20161209164625.GL3506@pathway.suse.cz> <20161210031022.GA493@tigerII.localdomain> <20161212135418.GN3506@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161212135418.GN3506@pathway.suse.cz> User-Agent: Mutt/1.7.2 (2016-11-26) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (12/12/16 14:54), Petr Mladek wrote: > On Sat 2016-12-10 12:10:22, Sergey Senozhatsky wrote: > > On (12/09/16 17:46), Petr Mladek wrote: > > > > -/* > > > > - * Safe printk() for NMI context. It uses a per-CPU buffer to > > > > - * store the message. NMIs are not nested, so there is always only > > > > - * one writer running. But the buffer might get flushed from another > > > > - * CPU, so we need to be careful. > > > > - */ > > > > > > We should keep/create a good description here because the function > > > has a non-trivial code. What about something like? > > > > > > > which is really not related to this patch set. > > I am sorry but I do not understand. This patch removes description > that explained constrains of a rather complex code. In fact, the > constrains has changed because we started using the function also > in other context. When will be the right time/patchset to explain > it? but I didn't remove it. $ grep -A3 -B3 'But the buffer might get flushed from another' kernel/printk/printk_safe.c /* * Safe printk() for NMI context. It uses a per-CPU buffer to * store the message. NMIs are not nested, so there is always only * one writer running. But the buffer might get flushed from another * CPU, so we need to be careful. */ static int vprintk_safe_nmi(const char *fmt, va_list args) > > > > +#ifdef CONFIG_PRINTK_NMI > > > > +/* > > > > + * Safe printk() for NMI context. It uses a per-CPU buffer to > > > > + * store the message. NMIs are not nested, so there is always only > > > > + * one writer running. But the buffer might get flushed from another > > > > + * CPU, so we need to be careful. > > > > + */ > > > > > > Hmm, I wanted to describe why we need another per-CPU buffer in NMI > > > and I am not sure that we really need it. > > > > NMI-printk can interrupt safe-printk's vsnprintf() in the middle of > > the "while (*fmt)" loop: safe-priNMI-PRINTK > > But this already happens when any of the WARNs is triggered > inside vsnprintf(). Either this is safe or we are in > trouble. the point was that when printk-safe resumes after being interrupted by NMI-printk it continues printing from the offset at which it has been interrupted, writing over the lines that were sprintf-d by NMI printk; because NMI-printk used the same buffer offset `s->len'. so at least part of NMI-printk message will be lost. -ss