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=-15.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 86EDBC433DB for ; Mon, 1 Feb 2021 14:15:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4B49364EA3 for ; Mon, 1 Feb 2021 14:15:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232196AbhBAOPt (ORCPT ); Mon, 1 Feb 2021 09:15:49 -0500 Received: from mx2.suse.de ([195.135.220.15]:33360 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231284AbhBAOPr (ORCPT ); Mon, 1 Feb 2021 09:15:47 -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=1612188901; 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=KeVl/uzEZKfN6JgWary/epj1tkH9HJSzmu1c+QHyWZM=; b=XpnfmTTBFEQMJcse5aLbgofgX0qKhg6Tm1v5Kan32b7Weob53ydJOd0R8Mh7N5IwAWZFvO xFaUMCxcgfWDMdt/PgNJi6tscsoz/wGcejRcPZlBl0Nrd/QnuH8zheSCVRyONg/TRQeF9z ElDaxgR03BH7z260u7EHxF+v4y0ycXA= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 7D801ABD5; Mon, 1 Feb 2021 14:15:01 +0000 (UTC) Date: Mon, 1 Feb 2021 15:15:00 +0100 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper Message-ID: References: <20210126211551.26536-1-john.ogness@linutronix.de> <20210126211551.26536-10-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210126211551.26536-10-john.ogness@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2021-01-26 22:21:48, John Ogness wrote: > In preparation for removing printk's @logbuf_lock, dumpers that have > assumed to be protected against parallel calls must provide their own > synchronization. Add a locally static spinlock to synchronize the > kmsg_dump call and temporary buffer usage. > > Signed-off-by: John Ogness > --- > arch/um/kernel/kmsg_dump.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c > index f38349ad00ea..173999422ed8 100644 > --- a/arch/um/kernel/kmsg_dump.c > +++ b/arch/um/kernel/kmsg_dump.c > @@ -1,5 +1,6 @@ > // SPDX-License-Identifier: GPL-2.0 > #include > +#include > #include > #include > #include > @@ -9,8 +10,10 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper, > enum kmsg_dump_reason reason, > struct kmsg_dumper_iter *iter) > { > + static DEFINE_SPINLOCK(lock); > static char line[1024]; > struct console *con; > + unsigned long flags; > size_t len = 0; > > /* only dump kmsg when no console is available */ > @@ -25,11 +28,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper, > if (con) > return; > > + if (!spin_trylock_irqsave(&lock, flags)) > + return; > + > printf("kmsg_dump:\n"); > while (kmsg_dump_get_line(iter, true, line, sizeof(line), &len)) { > line[len] = '\0'; > printf("%s", line); > } > + > + spin_unlock_irqrestore(&lock, flags); What exactly is synchronized here, please? Access to @line buffer or @iter or both? It looks to me that the access to @line buffer was not synchronized before. kmsg_dump_get_line() used a lock internally but it was not synchronized with the later printf("%s", line); The only remaining reason might be the access to @iter. But it is on stack in kmsg_dump(). Nobody else could have access to it. IMHO, this patch is not needed. Do I miss something, please? Best Regards, Petr