From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: [Xen-devel] printk from softirq on xen: hard lockup Date: Thu, 6 Aug 2015 09:22:49 +0100 Message-ID: <1438849369.9747.91.camel@citrix.com> References: <55C0F291.7060806@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Paul McKenney To: David Vrabel , "Jason A. Donenfeld" , , , Return-path: In-Reply-To: <55C0F291.7060806@citrix.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, 2015-08-04 at 18:12 +0100, David Vrabel wrote: > On 04/08/15 17:41, Jason A. Donenfeld wrote: > > Hi folks, > > > > Paul McKenney and I had an offline discussion about some rcu questions > > that eventually lead into me investigating a strange full lock-up I'm > > experiencing as a consequence of a printk in softirq inside of an > > rcu_read_lock, when using Xen PV. Relevant excerpts of the > ^^ PV guest? > > > (gdb) target remote localhost:9999 > > Remote debugging using localhost:9999 > > __xapic_wait_icr_idle () at ./arch/x86/include/asm/ipi.h:56 > > 56 while (native_apic_mem_read(APIC_ICR) & APIC_ICR_BUSY) > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ => HVM guest > > Which is it? Aren't there still some code paths for PV guests which hit the native APIC case (emulated in Xen even for PV these days, since pvops in the early days didn't accept the hooks needed to make use of the hypercall versions of apic read/write). In particular I'm thinking of the IPI which is (or was) used by the sysrq to trigger the backtrace on all CPUs. Ian.