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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 3D22BC43603 for ; Wed, 4 Dec 2019 12:54:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 152D420674 for ; Wed, 4 Dec 2019 12:54:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727779AbfLDMyz (ORCPT ); Wed, 4 Dec 2019 07:54:55 -0500 Received: from mx2.suse.de ([195.135.220.15]:36264 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726010AbfLDMyy (ORCPT ); Wed, 4 Dec 2019 07:54:54 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 29F11B202; Wed, 4 Dec 2019 12:54:52 +0000 (UTC) Date: Wed, 4 Dec 2019 13:54:50 +0100 From: Petr Mladek To: John Ogness Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Greg Kroah-Hartman , Andrea Parri , Thomas Gleixner , Sergey Senozhatsky , Brendan Higgins , kexec@lists.infradead.org Subject: Re: [RFC PATCH v5 2/3] printk-rb: new printk ringbuffer implementation (reader) Message-ID: <20191204125450.ob5b7xi3gevor4qz@pathway.suse.cz> References: <20191128015235.12940-1-john.ogness@linutronix.de> <20191128015235.12940-3-john.ogness@linutronix.de> <20191203120622.zux33do54rmjafns@pathway.suse.cz> <87pnh5bjz4.fsf@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pnh5bjz4.fsf@linutronix.de> User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2019-12-03 14:46:07, John Ogness wrote: > On 2019-12-03, Petr Mladek wrote: > >> Add the reader implementation for the new ringbuffer. > >> > >> Signed-off-by: John Ogness > >> --- > >> kernel/printk/printk_ringbuffer.c | 234 ++++++++++++++++++++++++++++++ > >> kernel/printk/printk_ringbuffer.h | 12 +- > >> 2 files changed, 245 insertions(+), 1 deletion(-) > >> > >> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c > >> index 09c32e52fd40..f85762713583 100644 > >> --- a/kernel/printk/printk_ringbuffer.c > >> +++ b/kernel/printk/printk_ringbuffer.c > >> @@ -674,3 +674,237 @@ void prb_commit(struct prb_reserved_entry *e) > >> local_irq_restore(e->irqflags); > >> } > >> EXPORT_SYMBOL(prb_commit); > >> + > >> +/* > >> + * Given @blk_lpos, return a pointer to the raw data from the data block > >> + * and calculate the size of the data part. A NULL pointer is returned > >> + * if @blk_lpos specifies values that could never be legal. > >> + * > >> + * This function (used by readers) performs strict validation on the lpos > >> + * values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is > >> + * triggered if an internal error is detected. > >> + */ > >> +static char *get_data(struct prb_data_ring *data_ring, > >> + struct prb_data_blk_lpos *blk_lpos, > >> + unsigned long *data_size) > >> +{ > >> + struct prb_data_block *db; > >> + > >> + if (blk_lpos->begin == INVALID_LPOS && > >> + blk_lpos->next == INVALID_LPOS) { > >> + /* descriptor without a data block */ > >> + return NULL; > >> + } else if (DATA_WRAPS(data_ring, blk_lpos->begin) == > >> + DATA_WRAPS(data_ring, blk_lpos->next)) { > >> + /* regular data block */ > >> + if (WARN_ON_ONCE(blk_lpos->next <= blk_lpos->begin)) > >> + return NULL; > >> + db = to_block(data_ring, blk_lpos->begin); > >> + *data_size = blk_lpos->next - blk_lpos->begin; > >> + > >> + } else if ((DATA_WRAPS(data_ring, blk_lpos->begin) + 1 == > >> + DATA_WRAPS(data_ring, blk_lpos->next)) || > >> + ((DATA_WRAPS(data_ring, blk_lpos->begin) == > >> + DATA_WRAPS(data_ring, -1UL)) && > >> + (DATA_WRAPS(data_ring, blk_lpos->next) == 0))) { > > > > I am a bit confused. I would expect that (-1UL + 1) = 0. So the second > > condition after || looks just like a special variant of the first > > valid condition. > > > > Or do I miss anything? Is there a problems with type casting? > > Sorry, this code deserves a comment. > > Here we are only comparing the number of wraps. For a wrapping data > block, @begin will be 1 wrap less than @next. The first part of the > check is checking the typical case, making sure that: > > 1 + WRAPS(@begin) == WRAPS(@next) > > There is also the case when the lpos overflows. In that case the number > of wraps starts over at zero (without having overflowed). (Note: The > lpos overflows, _not_ the number of wraps. This is why the first check > is not enough.) In this case, the number of wraps of the highest > possible lpos value (-1UL) should be the same as the number of wraps of > @begin. And the number of wraps of @next should be 0. The simplified > pseudo-code check is: > > WRAPS(@begin) == WRAPS(-1UL) > && > WRAPS(@next) == 0 Got it. I knew that it must have been something like this but I did not see it. I wonder if the following might be easier to understand even for people like me ;-) } else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) == DATA_WRAPS(data_ring, blk_lpos->next)) { Best Regards, Petr