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=-2.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 E7F13C76190 for ; Mon, 15 Jul 2019 07:54:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BD0D0214C6 for ; Mon, 15 Jul 2019 07:54:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="FVaAFDtJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729327AbfGOHy6 (ORCPT ); Mon, 15 Jul 2019 03:54:58 -0400 Received: from mail-pl1-f195.google.com ([209.85.214.195]:40807 "EHLO mail-pl1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726170AbfGOHy5 (ORCPT ); Mon, 15 Jul 2019 03:54:57 -0400 Received: by mail-pl1-f195.google.com with SMTP id a93so7871879pla.7 for ; Mon, 15 Jul 2019 00:54:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=2EqLe3LAwEFcMwoHbzXmTACPyJeJ406ewu59arKYTaM=; b=FVaAFDtJuZG19DfBwFD7hlUv0ewIEkytt8qottjjH6yLFKGLQZUmqtu92pVUnxR+T7 OcymMZ2hW18LnQq9HLu1LPIrO7Z+u9v0I8vj3+gE2E9rqbjjZXeA/grpFTUVdaGQRP9D BIgVVX1dlFuJVALNj3JfQV+XjDQDcByO1nj3TsxlKwDpI1eudAuKEcVZ9k9eSu7PZi8Q MKcQ0GEAfd8cDbFrxWwdEHxmP9nenG19Y5FJEdeo/kDRsl5n2yqD0pXep6LyI8dHyl2c AUA+ixWxIXtyo1mEQByRj5WjiuO2b6xtMm5OUg4ldhoFxLVEAkXWccV9k8y/3CC3SEN3 F8kQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=2EqLe3LAwEFcMwoHbzXmTACPyJeJ406ewu59arKYTaM=; b=klwtQF6j3Rm11ts1CRqENogq3lvr90gHQPeM1YIlOArccLvobpG8bUCNBIttU9Zrsd I5mJ4GwFBu8357ccRtp3t87jnwQGXRbf6UvOyRBr/M20N1ysMpOg0rRBOj+1eFJvp73N kZrhK1FomOycNw9ORq4ldJLdVh7pAmNyefXH6pGghiNJsatK5jzhAJyj3PRIGYFpBqey iA5fv4LYOqwmOdnfaOu8s7OtOqES41GEamRrfR9eCwTcayf0xL5Vki1yrxAdaQD2x0pt VsZaR14vfHEk8BlLyRy94mE1twedoHpVXGyrP75innkt6qEsHLJ4hhxcQlPwm91SIttB WyLA== X-Gm-Message-State: APjAAAWZfvDcm6zzMPVOJxKYFjMtXKFvZ3xjyH0LwpUy6B+/1gqQRl/z ZFtu1WxpVYTnlQKsQwSYdBk= X-Google-Smtp-Source: APXvYqyvGX1c5xjAd2PUbd8cxy5fa7IPH68r/R9GT7KlHj5OcRJmtbddcpa/IxsmJz0m7D4/D2uKIg== X-Received: by 2002:a17:902:f301:: with SMTP id gb1mr26442352plb.292.1563177296950; Mon, 15 Jul 2019 00:54:56 -0700 (PDT) Received: from localhost ([39.7.59.60]) by smtp.gmail.com with ESMTPSA id t17sm18816843pgg.48.2019.07.15.00.54.55 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Mon, 15 Jul 2019 00:54:56 -0700 (PDT) Date: Mon, 15 Jul 2019 16:54:52 +0900 From: Sergey Senozhatsky To: Konstantin Khlebnikov Cc: Sergey Senozhatsky , Konstantin Khlebnikov , Sergey Senozhatsky , Petr Mladek , Linux Kernel Mailing List , Steven Rostedt Subject: Re: [PATCH] kernel/printk: prevent deadlock at calling kmsg_dump from NMI context Message-ID: <20190715075452.GA1875@jagdpanzerIV> References: <156294329676.1745.2620297516210526183.stgit@buzz> <20190713060929.GB1038@tigerII.localdomain> <20190713131947.GA4464@tigerII.localdomain> <20190715023338.GB3653@jagdpanzerIV> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (07/15/19 10:38), Konstantin Khlebnikov wrote: > > > Indeed, panic is especially handled and looks fine. > > > > > > Sanity check in my patch could be relaxed: > > > > > > if (WARN_ON_ONCE(reason != KMSG_DUMP_PANIC && in_nmi())) > > > return; > > > > How critical kmsg_dump() is? We deadlock only if NMI->kmsg_dump() > > happens on the CPU which already holds the logbuf_lock; in any > > other case logbuf_lock is owned by another CPU which is expected > > to unlock it eventually. So it doesn't look like disabling all > > NMI->kmsg_dump() is the only way to fix it. > > > > When we lock logbuf_lock we increment per-CPU printk_context > > (PRINTK_SAFE_CONTEXT_MASK bits); when we unlock logbuf_lock > > we decrement printk_context. Thus we always can tell if the > > logbuf_lock was locked on the very same CPU - this_cpu printk_context > > has PRINTK_SAFE_CONTEXT_MASK bits sets - and we are about to deadlock > > in a nested context (NMI), or the lock was locked on another CPU and > > it's "safe" to spin on logbuf_lock and wait for logbuf_lock to become > > available. > > I see no users who dumps dmesg from NMI context except panic. > This shouldn't happen. But might happen is something goes very wrong. I see. OK, if NMI->kmsg_dump() never happens (except for panic) then we are fine, I guess. -ss