> From: linux-kernel-owner at vger.kernel.org [mailto:linux-kernel-owner at vger.kernel.org] On Behalf Of Xunlei Pang > Sent: Tuesday, July 12, 2016 3:57 PM > On 2016/07/12 at 11:56, ???? / KAWAI?HIDEHIRO wrote: > > Hi Xunlei, > > > > Thanks for the review. > > > >> From: Xunlei Pang [mailto:xpang at redhat.com] > >> Sent: Tuesday, July 12, 2016 12:12 PM > >> On 2016/07/05 at 19:33, Hidehiro Kawai wrote: > >>> This patch fixes one of the problems reported by Daniel Walker > >>> (https://lkml.org/lkml/2015/6/24/44). > >>> > >>> If crash_kexec_post_notifiers boot option is specified, other CPUs > >>> are stopped by smp_send_stop() instead of machine_crash_shutdown() > >>> in crash_kexec() path. This behavior change leads two problems. > >>> > >>> Problem 1: > >>> octeon_generic_shutdown() for MIPS OCTEON assumes that other CPUs are > >>> still online and try to stop their watchdog timer. If > >>> smp_send_stop() is called before octeon_generic_shutdown(), stopping > >>> watchdog timer will fail because other CPUs have been offlined by > >>> smp_send_stop(). > >>> > >>> panic() > >>> if crash_kexec_post_notifiers == 1 > >>> smp_send_stop() > >>> atomic_notifier_call_chain() > >>> kmsg_dump() > >>> crash_kexec() > >>> machine_crash_shutdown() > >>> octeon_generic_shutdown() // shutdown watchdog for ONLINE CPUs > >>> > >>> Problem 2: > >>> Most of architectures stop other CPUs in machine_crash_shutdown() > >>> path, and they also do something needed for kdump. For example, > >>> they save registers, disable virtualization extensions, and so on. > >>> However, if smp_send_stop() stops other CPUs before > >>> machine_crash_shutdown(), we miss those operations. > >>> > >>> How do we fix these problems? In the first place, we should stop > >>> other CPUs as soon as possible when panic() was called, otherwise > >>> other CPUs may wipe out a clue to the cause of the failure. So, we > >>> replace smp_send_stop() with more suitable one for kdump. > >>> > >>> This patch solves Problem 2 by replacing smp_send_stop() in panic() > >>> with panic_smp_send_stop(). This is a weak function which calls > >>> smp_send_stop(), and architecture dependent code may override this > >>> with appropriate one. This patch only provides x86-specific version. > >>> > >>> Changes in V3: > >>> - Revise comments, description, and symbol names > >>> > >>> Changes in V2: > >>> - Replace smp_send_stop() call with crash_kexec version which > >>> saves cpu states and cleans up VMX/SVM > >>> - Drop a fix for Problem 1 at this moment > >>> > >>> Reported-by: Daniel Walker <dwalker at fifo99.com> > >>> Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers" option) > >>> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez at hitachi.com> > >>> Cc: Dave Young <dyoung at redhat.com> > >>> Cc: Baoquan He <bhe at redhat.com> > >>> Cc: Vivek Goyal <vgoyal at redhat.com> > >>> Cc: Eric Biederman <ebiederm at xmission.com> > >>> Cc: Masami Hiramatsu <mhiramat at kernel.org> > >>> Cc: Thomas Gleixner <tglx at linutronix.de> > >>> Cc: Ingo Molnar <mingo at redhat.com> > >>> Cc: "H. Peter Anvin" <hpa at zytor.com> > >>> Cc: Borislav Petkov <bp at suse.de> > >>> Cc: Toshi Kani <toshi.kani at hpe.com> > >>> Cc: "Peter Zijlstra (Intel)" <peterz at infradead.org> > >>> Cc: Takao Indoh <indou.takao at jp.fujitsu.com> > >>> Cc: "Lee, Chun-Yi" <joeyli.kernel at gmail.com> > >>> Cc: Minfei Huang <mnfhuang at gmail.com> > >>> Cc: Andrew Morton <akpm at linux-foundation.org> > >>> Cc: Michal Hocko <mhocko at suse.com> > >>> Cc: Vitaly Kuznetsov <vkuznets at redhat.com> > >>> Cc: Petr Mladek <pmladek at suse.com> > >>> Cc: Tejun Heo <tj at kernel.org> > >>> Cc: Josh Poimboeuf <jpoimboe at redhat.com> > >>> --- > >>> arch/x86/kernel/crash.c | 14 ++++++++++---- > >>> kernel/panic.c | 43 ++++++++++++++++++++++++++++++++----------- > >>> 2 files changed, 42 insertions(+), 15 deletions(-) > >>> > >>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c > >>> index 9ef978d..3305433 100644 > >>> --- a/arch/x86/kernel/crash.c > >>> +++ b/arch/x86/kernel/crash.c > >>> @@ -133,15 +133,21 @@ static void kdump_nmi_callback(int cpu, struct pt_regs *regs) > >>> disable_local_APIC(); > >>> } > >>> > >>> -static void kdump_nmi_shootdown_cpus(void) > >>> +/* Override the weak function in kernel/panic.c */ > >>> +void panic_smp_send_stop(void) > >>> { > >>> - nmi_shootdown_cpus(kdump_nmi_callback); > >>> + static int cpus_stopped; > >> Should be atomic_t type? > > panic_smp_send_stop() can be called by only one panicking CPU > > (but can be called twice). It is sufficient to be normal variable. > > There are other call sites of __crash_kexec() for oops cases, which can > call panic_smp_send_stop() concurrently on a different cpu. In oops cases, crash_kexec() is called first, then __crash_kexec() is called. crash_kexec() excludes concurrent execution of panic() and crash_kexec() via panic_cpu, so panic_smp_send_stop() shouldn't be called concurrently. Best regards, Hidehiro Kawai Hitachi, Ltd. Research & Development Group
