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 B7F60C4332F for ; Wed, 8 Nov 2023 11:09:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344291AbjKHLJE (ORCPT ); Wed, 8 Nov 2023 06:09:04 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53396 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229579AbjKHLJC (ORCPT ); Wed, 8 Nov 2023 06:09:02 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 29A0D186 for ; Wed, 8 Nov 2023 03:09:00 -0800 (PST) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1699441737; h=from:from:reply-to:subject:subject: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=/9frZEkbYti4SPFm3gs1C2FC6vGu6r4Wj0LGB/iG23k=; b=kA99J7Nib6NUh+FF88jn21BkhUxV/zXwDNst5UWwj53gPsH1tii35i1Q5RkPvka/ABUPDA rgL1h8XyU/sLfHSZwguKTZTUnVJDVGQZUZ3jZUrN4Ltwn5UZg69ciJjIMQOwylA1ILoAEI sdhP900BJg4DbuINiFq/xVgAv7ySBJ+TMVGdRwuRu1upoLeXsqKMwrNCaSq1Ln/4YUDFgx jzclIwUsfH2I0HKqtRlR7BD4FLlfyy3WGqjFAB1Qo5WjISt2YYQBrVr9bFZhNRFIW4rk2H a9QrvfRby//LbZThOg/JvAJrWwIjZkKUsAZ81V2LZwFRUeUynF5dugMg/iEi8Q== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1699441737; h=from:from:reply-to:subject:subject: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=/9frZEkbYti4SPFm3gs1C2FC6vGu6r4Wj0LGB/iG23k=; b=tEkh049got+CEe2byYBEngXapEwMqmX5Wl/92bvP9DHeZmMUX/k3RZE9t3cxR7VLFySjyx OBPVEyfZX6NgiCAg== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk v2 6/9] printk: Wait for all reserved records with pr_flush() In-Reply-To: <20231106210730.115192-7-john.ogness@linutronix.de> References: <20231106210730.115192-1-john.ogness@linutronix.de> <20231106210730.115192-7-john.ogness@linutronix.de> Date: Wed, 08 Nov 2023 12:14:54 +0106 Message-ID: <87h6lwcxix.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023-11-06, John Ogness wrote: > diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c > index 7c121fd68330..dc83569d3a3a 100644 > --- a/kernel/printk/printk_ringbuffer.c > +++ b/kernel/printk/printk_ringbuffer.c > @@ -2012,6 +2012,102 @@ static u64 prb_first_seq(struct printk_ringbuffer *rb) > return seq; > } > > +/** > + * prb_next_reserve_seq() - Get the sequence number after the most recently > + * reserved record. > + * > + * @rb: The ringbuffer to get the sequence number from. > + * > + * This is the public function available to readers to see what sequence > + * number will be assigned to the next reserved record. > + * > + * Note that depending on the situation, this value can be equal to or > + * higher than the sequence number returned by prb_next_seq(). > + * > + * Context: Any context. > + * Return: The sequence number that will be assigned to the next record > + * reserved. > + */ > +u64 prb_next_reserve_seq(struct printk_ringbuffer *rb) > +{ > + struct prb_desc_ring *desc_ring = &rb->desc_ring; > + unsigned long last_finalized_id; > + atomic_long_t *state_var; > + u64 last_finalized_seq; > + unsigned long head_id; > + struct prb_desc desc; > + unsigned long diff; > + struct prb_desc *d; > + int err; > + > + /* > + * It may not be possible to read a sequence number for @head_id. > + * So the ID of @last_finailzed_seq is used to calculate what the > + * sequence number of @head_id will be. > + */ > + > +try_again: > + last_finalized_seq = desc_last_finalized_seq(desc_ring); > + > + /* > + * @head_id is loaded after @last_finalized_seq to ensure that it is > + * at or beyond @last_finalized_seq. > + * > + * Memory barrier involvement: > + * > + * If desc_last_finalized_seq:A reads from > + * desc_update_last_finalized:A, then > + * prb_next_reserve_seq:A reads from desc_reserve:D. > + * > + * Relies on: > + * > + * RELEASE from desc_reserve:D to desc_update_last_finalized:A > + * matching > + * ACQUIRE from desc_last_finalized_seq:A to prb_next_reserve_seq:A > + * > + * Note: desc_reserve:D and desc_update_last_finalized:A can be > + * different CPUs. However, the desc_update_last_finalized:A CPU > + * (which performs the release) must have previously seen > + * desc_read:C, which implies desc_reserve:D can be seen. > + */ > + head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_next_reserve_seq:A) */ > + > + d = to_desc(desc_ring, last_finalized_seq); > + state_var = &d->state_var; > + > + /* Extract the ID, used to specify the descriptor to read. */ > + last_finalized_id = DESC_ID(atomic_long_read(state_var)); > + > + /* Ensure @last_finalized_id is correct. */ > + err = desc_read_finalized_seq(desc_ring, last_finalized_id, last_finalized_seq, &desc); > + > + if (err == -EINVAL) { > + if (last_finalized_seq == 0) { > + /* > + * @last_finalized_seq still contains its initial > + * value. Probably no record has been finalized yet. > + * This means the ringbuffer is not yet full and the > + * @head_id value can be used directly (subtracting > + * off its initial value). > + * > + * Because of hack#2 of the bootstrapping phase, the > + * @head_id initial value must be handled separately. > + */ > + if (head_id == DESC0_ID(desc_ring->count_bits)) > + return 0; > + > + last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1; > + } else { > + /* Record must have been overwritten. Try again. */ > + goto try_again; > + } > + } > + > + diff = head_id - last_finalized_id; > + > + return (last_finalized_seq + diff); This needs to be: return (last_finalized_seq + diff + 1); because @last_finalized_seq and @head_id refer to existing records, but this function should return the following (not yet existing) record. John Ogness