On 2016/07/12 at 11:56, 河合英宏 / KAWAI，HIDEHIRO wrote:
> Hi Xunlei,
>
> Thanks for the review.
>
>> From: Xunlei Pang [mailto:xp...@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 <dwal...@fifo99.com>
>>> Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers"
>>> option)
>>> Signed-off-by: Hidehiro Kawai <hidehiro.kawai...@hitachi.com>
>>> Cc: Dave Young <dyo...@redhat.com>
>>> Cc: Baoquan He <b...@redhat.com>
>>> Cc: Vivek Goyal <vgo...@redhat.com>
>>> Cc: Eric Biederman <ebied...@xmission.com>
>>> Cc: Masami Hiramatsu <mhira...@kernel.org>
>>> Cc: Thomas Gleixner <t...@linutronix.de>
>>> Cc: Ingo Molnar <mi...@redhat.com>
>>> Cc: "H. Peter Anvin" <h...@zytor.com>
>>> Cc: Borislav Petkov <b...@suse.de>
>>> Cc: Toshi Kani <toshi.k...@hpe.com>
>>> Cc: "Peter Zijlstra (Intel)" <pet...@infradead.org>
>>> Cc: Takao Indoh <indou.ta...@jp.fujitsu.com>
>>> Cc: "Lee, Chun-Yi" <joeyli.ker...@gmail.com>
>>> Cc: Minfei Huang <mnfhu...@gmail.com>
>>> Cc: Andrew Morton <a...@linux-foundation.org>
>>> Cc: Michal Hocko <mho...@suse.com>
>>> Cc: Vitaly Kuznetsov <vkuzn...@redhat.com>
>>> Cc: Petr Mladek <pmla...@suse.com>
>>> Cc: Tejun Heo <t...@kernel.org>
>>> Cc: Josh Poimboeuf <jpoim...@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.
Regards,
Xunlei
>>> +
>>> + if (cpus_stopped)
>>> + return;
>>>
>>> + nmi_shootdown_cpus(kdump_nmi_callback);
>>> disable_local_APIC();
>>> + cpus_stopped = 1;
>>> }
>>>
>>> #else
>>> -static void kdump_nmi_shootdown_cpus(void)
>>> +void panic_smp_send_stop(void)
>>> {
>>> /* There are no cpus to shootdown */
>>> }
>>> @@ -160,7 +166,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>>> /* The kernel is broken so disable interrupts */
>>> local_irq_disable();
>>>
>>> - kdump_nmi_shootdown_cpus();
>>> + panic_smp_send_stop();
>>>
>>> /*
>>> * VMCLEAR VMCSs loaded on this cpu if needed.
>>> diff --git a/kernel/panic.c b/kernel/panic.c
>>> index 8aa7449..da8062d2 100644
>>> --- a/kernel/panic.c
>>> +++ b/kernel/panic.c
>>> @@ -71,6 +71,32 @@ void __weak nmi_panic_self_stop(struct pt_regs *regs)
>>> panic_smp_self_stop();
>>> }
>>>
>>> +/*
>>> + * Stop other CPUs in panic. Architecture dependent code may override this
>>> + * with more suitable version. For example, if the architecture supports
>>> + * crash dump, it should save registers of each stopped CPU and disable
>>> + * per-CPU features such as virtualization extensions.
>>> + */
>>> +void __weak panic_smp_send_stop(void)
>>> +{
>>> + static int cpus_stopped;
>>> +
>>> + /*
>>> + * This function can be called twice in panic path, but obviously
>>> + * we execute this only once.
>>> + */
>>> + if (cpus_stopped)
>>> + return;
>>> +
>>> + /*
>>> + * Note smp_send_stop is the usual smp shutdown function, which
>>> + * unfortunately means it may not be hardened to work in a panic
>>> + * situation.
>>> + */
>>> + smp_send_stop();
>>> + cpus_stopped = 1;
>>> +}
>>> +
>>> atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
>>>
>>> /*
>>> @@ -125,7 +151,7 @@ void panic(const char *fmt, ...)
>>> * Only one CPU is allowed to execute the panic code from here. For
>>> * multiple parallel invocations of panic, all other CPUs either
>>> * stop themself or will wait until they are stopped by the 1st CPU
>>> - * with smp_send_stop().
>>> + * with panic_smp_send_stop().
>>> *
>>> * `old_cpu == PANIC_CPU_INVALID' means this is the 1st CPU which
>>> * comes here, so go ahead.
>>> @@ -165,12 +191,7 @@ void panic(const char *fmt, ...)
>>> __crash_kexec(NULL);
>>> }
>>>
>>> - /*
>>> - * Note smp_send_stop is the usual smp shutdown function, which
>>> - * unfortunately means it may not be hardened to work in a panic
>>> - * situation.
>>> - */
>>> - smp_send_stop();
>>> + panic_smp_send_stop();
>> If crash_kexec_post_notifiers and kdump both are not used, you actually
>> changed the original behaviour,
>> it would be better to make it conditionally like:
>> if (crash_kexec_post_notifiers)
>> panic_smp_send_stop(); // renaming it to crash_smp_send_stop() is
>> better?
>> else
>> smp_send_stop();
> Dave also pointed about this, and I'll fix it.
>
> Best regards,
>
> Hidehiro Kawai
> Hitachi, Ltd. Research & Development Group
>
>>> /*
>>> * Run any panic handlers, including those that might need to
>>> @@ -198,10 +219,10 @@ void panic(const char *fmt, ...)
>>>
>>> /*
>>> * We may have ended up stopping the CPU holding the lock (in
>>> - * smp_send_stop()) while still having some valuable data in the console
>>> - * buffer. Try to acquire the lock then release it regardless of the
>>> - * result. The release will also print the buffers out. Locks debug
>>> - * should be disabled to avoid reporting bad unlock balance when
>>> + * panic_smp_send_stop()) while still having some valuable data in the
>>> + * console buffer. Try to acquire the lock then release it regardless
>>> + * of the result. The release will also print the buffers out. Locks
>>> + * debug should be disabled to avoid reporting bad unlock balance when
>>> * panic() is not being callled from OOPS.
>>> */
>>> debug_locks_off();
>>>
>>>
>>>
>>> _______________________________________________
>>> kexec mailing list
>>> ke...@lists.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/kexec
