Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757518AbbLBCpV (ORCPT ); Tue, 1 Dec 2015 21:45:21 -0500 Received: from ozlabs.org ([103.22.144.67]:51365 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757380AbbLBCpT (ORCPT ); Tue, 1 Dec 2015 21:45:19 -0500 Message-ID: <1449024316.11810.6.camel@ellerman.id.au> Subject: Re: [PATCH v2 1/5] printk/nmi: Generic solution for safe printk in NMI From: Michael Ellerman To: Petr Mladek , Andrew Morton Cc: linux-mips@linux-mips.org, Daniel Thompson , Jiri Kosina , linux-cris-kernel@axis.com, linux-s390@vger.kernel.org, Peter Zijlstra , x86@kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt , adi-buildroot-devel@lists.sourceforge.net, Ingo Molnar , linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, Russell King , Thomas Gleixner , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org Date: Wed, 02 Dec 2015 13:45:16 +1100 In-Reply-To: <1448622572-16900-2-git-send-email-pmladek@suse.com> References: <1448622572-16900-1-git-send-email-pmladek@suse.com> <1448622572-16900-2-git-send-email-pmladek@suse.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5-1ubuntu3 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1365 Lines: 49 On Fri, 2015-11-27 at 12:09 +0100, Petr Mladek wrote: > printk() takes some locks and could not be used a safe way in NMI > context. > > The chance of a deadlock is real especially when printing > stacks from all CPUs. This particular problem has been addressed > on x86 by the commit a9edc8809328 ("x86/nmi: Perform a safe NMI stack > trace on all CPUs"). ... > diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c > new file mode 100644 > index 000000000000..3989e13a0021 > --- /dev/null > +++ b/kernel/printk/nmi.c > @@ -0,0 +1,200 @@ ... > + > +struct nmi_seq_buf { > + atomic_t len; /* length of written data */ > + struct irq_work work; /* IRQ work that flushes the buffer */ > + unsigned char buffer[PAGE_SIZE - sizeof(atomic_t) - > + sizeof(struct irq_work)]; > +}; > +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq); PAGE_SIZE isn't always 4K. On typical powerpc systems this will give you 128K, and on some 512K, which is probably not what we wanted. The existing code just did: #define NMI_BUF_SIZE 4096 So I think you should just go back to doing that. cheers -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/