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[V3 PATCH 0/4] Fix race issues among panic, NMI and crash_kexec

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  [V3 PATCH 0/4] Fix race issues among panic, NMI and crash_kexec Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> - 2015-08-06 07:50 +0200
    [V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()  called directly Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> - 2015-08-06 07:50 +0200
    [V3 PATCH 2/4] panic/x86: Allow cpus to save registers even if they  are looping in NMI context Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> - 2015-08-06 07:50 +0200
    [V3 PATCH 4/4] x86/apic: Introduce noextnmi boot option Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> - 2015-08-06 07:50 +0200
    Re: [V3 PATCH 0/4] Fix race issues among panic, NMI and crash_kexec Michal Hocko <mhocko@kernel.org> - 2015-08-07 16:40 +0200

#1201439 — [V3 PATCH 0/4] Fix race issues among panic, NMI and crash_kexec

When an HA cluster software or administrator detects non-response
of a host, they issue an NMI to the host to completely stop current
works and take a crash dump.  If the kernel has already panicked
or is capturing a crash dump at that time, further NMI can cause
a crash dump failure.

Also, crash_kexec() called from oops context and panic() can
cause race conditions.

To solve these issue, this patch set does following things:

- Don't panic on NMI if the kernel has already panicked
- Extend exclusion control currently done by panic_lock to crash_kexec
- Introduce "noextnmi" boot option which masks external NMI at the
  boot time (supported only for x86)

V3:
- Introduce nmi_panic() macro to reduce code duplication
- In the case of panic on NMI, don't return from NMI handlers
  if another cpu already panicked

V2:
- Use atomic_cmpxchg() instead of current spin_trylock() to exclude
  concurrent accesses to panic() and crash_kexec()
- Don't introduce no-lock version of panic() and crash_kexec()

---

Hidehiro Kawai (4):
      panic/x86: Fix re-entrance problem due to panic on NMI
      panic/x86: Allow cpus to save registers even if they are looping in NMI context
      kexec: Fix race between panic() and crash_kexec() called directly
      x86/apic: Introduce noextnmi boot option


 Documentation/kernel-parameters.txt |    4 ++++
 arch/x86/kernel/apic/apic.c         |   17 ++++++++++++++++-
 arch/x86/kernel/nmi.c               |   15 +++++++++++----
 arch/x86/kernel/reboot.c            |   11 +++++++++++
 include/linux/kernel.h              |   21 +++++++++++++++++++++
 kernel/kexec.c                      |   20 ++++++++++++++++++++
 kernel/panic.c                      |   23 ++++++++++++++++++++---
 kernel/watchdog.c                   |    5 +++--
 8 files changed, 106 insertions(+), 10 deletions(-)


-- 
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group


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#1201440 — [V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

Currently, panic() and crash_kexec() can be called at the same time.
For example (x86 case):

CPU 0:
  oops_end()
    crash_kexec()
      mutex_trylock() // acquired
        nmi_shootdown_cpus() // stop other cpus

CPU 1:
  panic()
    crash_kexec()
      mutex_trylock() // failed to acquire
    smp_send_stop() // stop other cpus
    infinite loop

If CPU 1 calls smp_send_stop() before nmi_shootdown_cpus(), kdump
fails.

In another case:

CPU 0:
  oops_end()
    crash_kexec()
      mutex_trylock() // acquired
        <NMI>
        io_check_error()
          panic()
            crash_kexec()
              mutex_trylock() // failed to acquire
            infinite loop

Clearly, this is an undesirable result.

To fix this problem, this patch changes crash_kexec() to exclude
others by using atomic_t panicking_cpu.

V2:
- Use atomic_cmpxchg() instead of spin_trylock() on panic_lock
  to exclude concurrent accesses
- Don't introduce no-lock version of crash_kexec()

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
---
 kernel/kexec.c |   20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/kernel/kexec.c b/kernel/kexec.c
index a785c10..ed75a8b 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1472,6 +1472,18 @@ void __weak crash_unmap_reserved_pages(void)
 
 void crash_kexec(struct pt_regs *regs)
 {
+	int old_cpu, this_cpu;
+
+	/*
+	 * `old_cpu == -1' means we are the first comer and crash_kexec()
+	 * was called without entering panic().
+	 * `old_cpu == this_cpu' means crash_kexec() was called from panic().
+	 */
+	this_cpu = raw_smp_processor_id();
+	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
+	if (old_cpu != -1 && old_cpu != this_cpu)
+		return;
+
 	/* Take the kexec_mutex here to prevent sys_kexec_load
 	 * running on one cpu from replacing the crash kernel
 	 * we are using after a panic on a different cpu.
@@ -1491,6 +1503,14 @@ void crash_kexec(struct pt_regs *regs)
 		}
 		mutex_unlock(&kexec_mutex);
 	}
+
+	/*
+	 * If we came here from panic(), we have to keep panic_cpu
+	 * to prevent other cpus from entering panic().  Otherwise,
+	 * resetting it so that other cpus can enter panic()/crash_kexec().
+	 */
+	if (old_cpu == -1)
+		atomic_xchg(&panic_cpu, -1);
 }
 
 size_t crash_get_memory_size(void)


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#1201443 — [V3 PATCH 2/4] panic/x86: Allow cpus to save registers even if they are looping in NMI context

When cpu-A panics on NMI just after cpu-B has panicked, cpu-A loops
infinitely in NMI context.  Especially for x86, cpu-B issues NMI IPI
to other cpus to save their register states and do some cleanups if
kdump is enabled, but cpu-A can't handle the NMI and fails to save
register states.

To solve thie issue, we wait for the timing of the NMI IPI, then
call the NMI handler which saves register states.

V3:
- Newly introduced

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
---
 arch/x86/kernel/nmi.c    |    6 +++---
 arch/x86/kernel/reboot.c |   11 +++++++++++
 include/linux/kernel.h   |   12 ++++++++++--
 kernel/panic.c           |   10 ++++++++++
 kernel/watchdog.c        |    5 +++--
 5 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index dcd4038..fbb1877 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -231,7 +231,7 @@ void unregister_nmi_handler(unsigned int type, const char *name)
 #endif
 
 	if (panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 
@@ -256,7 +256,7 @@ void unregister_nmi_handler(unsigned int type, const char *name)
 	show_regs(regs);
 
 	if (panic_on_io_nmi) {
-		nmi_panic("NMI IOCK error: Not continuing");
+		nmi_panic(regs, "NMI IOCK error: Not continuing");
 
 		/*
 		 * If we return from here, we've already being in panic.
@@ -304,7 +304,7 @@ void unregister_nmi_handler(unsigned int type, const char *name)
 
 	pr_emerg("Do you have a strange power saving mode enabled?\n");
 	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
-		nmi_panic("NMI: Not continuing");
+		nmi_panic(regs, "NMI: Not continuing");
 
 	pr_emerg("Dazed and confused, but trying to continue\n");
 }
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 86db4bc..299b4b7 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -718,6 +718,7 @@ void machine_crash_shutdown(struct pt_regs *regs)
 static nmi_shootdown_cb shootdown_callback;
 
 static atomic_t waiting_for_crash_ipi;
+static int crash_ipi_done;
 
 static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
 {
@@ -779,6 +780,7 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 	wmb();
 
 	smp_send_nmi_allbutself();
+	crash_ipi_done = 1; /* Kick cpus looping in nmi context */
 
 	msecs = 1000; /* Wait at most a second for the other cpus to stop */
 	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
@@ -788,6 +790,15 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 
 	/* Leave the nmi callback set */
 }
+
+void nmi_panic_self_stop(struct pt_regs *regs)
+{
+	while (crash_ipi_done == 0)
+		cpu_relax();
+
+	crash_nmi_callback(0, regs); /* Shouldn't return */
+}
+
 #else /* !CONFIG_SMP */
 void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 {
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 57c33da..9fe9961 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -255,6 +255,7 @@ static inline void might_fault(void) { }
 __printf(1, 2)
 void panic(const char *fmt, ...)
 	__noreturn __cold;
+void nmi_panic_self_stop(struct pt_regs *);
 extern void oops_enter(void);
 extern void oops_exit(void);
 void print_oops_end_marker(void);
@@ -448,12 +449,19 @@ extern __scanf(2, 0)
 /*
  * A variant of panic() called from NMI context.
  * If we've already panicked on this cpu, return from here.
+ * If another cpu already panicked, loop in nmi_panic_self_stop() which
+ * can provide architecture dependent code such as saving register states
+ * for crash dump.
  */
-#define nmi_panic(fmt, ...)						\
+#define nmi_panic(regs, fmt, ...)					\
 	do {								\
+		int old_cpu;						\
 		int this_cpu = raw_smp_processor_id();			\
-		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
+		old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);	\
+		if (old_cpu == -1)					\
 			panic(fmt, ##__VA_ARGS__);			\
+		else if (old_cpu != this_cpu)				\
+			nmi_panic_self_stop(regs);			\
 	} while (0)
 
 /*
diff --git a/kernel/panic.c b/kernel/panic.c
index 3d4bc73..415c4e7 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -60,6 +60,16 @@ void __weak panic_smp_self_stop(void)
 		cpu_relax();
 }
 
+/*
+ * Stop ourself in NMI context if another cpu has already panicked.
+ * Architecture code may override this to prepare for crash dumping
+ * (e.g. save register information).
+ */
+void __weak nmi_panic_self_stop(struct pt_regs *regs)
+{
+	panic_smp_self_stop();
+}
+
 atomic_t panic_cpu = ATOMIC_INIT(-1);
 
 /**
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 9a3d738..8e3a31c 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -304,8 +304,9 @@ static void watchdog_overflow_callback(struct perf_event *event,
 			return;
 
 		if (hardlockup_panic)
-			nmi_panic("Watchdog detected hard LOCKUP on cpu %d",
-			      this_cpu);
+			nmi_panic(regs,
+				  "Watchdog detected hard LOCKUP on cpu %d",
+				  this_cpu);
 		else
 			WARN(1, "Watchdog detected hard LOCKUP on cpu %d",
 			     this_cpu);


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#1201449 — [V3 PATCH 4/4] x86/apic: Introduce noextnmi boot option

This patch introduces new boot option "noextnmi" which disables
external NMI.  This option is useful for the dump capture kernel
so that an HA application or administrator wouldn't mistakenly
shoot down the kernel by NMI.

Currently, only x86 supports this option.

Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
---
 Documentation/kernel-parameters.txt |    4 ++++
 arch/x86/kernel/apic/apic.c         |   17 ++++++++++++++++-
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 1d6f045..2cbd40b 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2364,6 +2364,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			noexec=on: enable non-executable mappings (default)
 			noexec=off: disable non-executable mappings
 
+	noextnmi	[X86]
+			Mask external NMI.  This option is useful for a
+			dump capture kernel to be shot down by NMI.
+
 	nosmap		[X86]
 			Disable SMAP (Supervisor Mode Access Prevention)
 			even if it is supported by processor.
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index dcb5285..a140410 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -82,6 +82,12 @@
 static unsigned int disabled_cpu_apicid __read_mostly = BAD_APICID;
 
 /*
+ * Don't enable external NMI via LINT1 on BSP.  This is useful for
+ * the dump capture kernel.
+ */
+static bool apic_noextnmi;
+
+/*
  * Map cpu index to physical APIC ID
  */
 DEFINE_EARLY_PER_CPU_READ_MOSTLY(u16, x86_cpu_to_apicid, BAD_APICID);
@@ -1150,6 +1156,8 @@ void __init init_bsp_APIC(void)
 	value = APIC_DM_NMI;
 	if (!lapic_is_integrated())		/* 82489DX */
 		value |= APIC_LVT_LEVEL_TRIGGER;
+	if (apic_noextnmi)
+		value |= APIC_LVT_MASKED;
 	apic_write(APIC_LVT1, value);
 }
 
@@ -1369,7 +1377,7 @@ void setup_local_APIC(void)
 	/*
 	 * only the BP should see the LINT1 NMI signal, obviously.
 	 */
-	if (!cpu)
+	if (!cpu && !apic_noextnmi)
 		value = APIC_DM_NMI;
 	else
 		value = APIC_DM_NMI | APIC_LVT_MASKED;
@@ -2537,3 +2545,10 @@ static int __init apic_set_disabled_cpu_apicid(char *arg)
 	return 0;
 }
 early_param("disable_cpu_apicid", apic_set_disabled_cpu_apicid);
+
+static int __init apic_set_noextnmi(char *arg)
+{
+	apic_noextnmi = true;
+	return 0;
+}
+early_param("noextnmi", apic_set_noextnmi);


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#1202752

JFYI I have those patches tested on the largish box. Will come back to
you as soon as I have some feedback.

I will also try to review these patches sometimes next week.

Thanks!

On Thu 06-08-15 14:45:43, Hidehiro Kawai wrote:
> When an HA cluster software or administrator detects non-response
> of a host, they issue an NMI to the host to completely stop current
> works and take a crash dump.  If the kernel has already panicked
> or is capturing a crash dump at that time, further NMI can cause
> a crash dump failure.
> 
> Also, crash_kexec() called from oops context and panic() can
> cause race conditions.
> 
> To solve these issue, this patch set does following things:
> 
> - Don't panic on NMI if the kernel has already panicked
> - Extend exclusion control currently done by panic_lock to crash_kexec
> - Introduce "noextnmi" boot option which masks external NMI at the
>   boot time (supported only for x86)
> 
> V3:
> - Introduce nmi_panic() macro to reduce code duplication
> - In the case of panic on NMI, don't return from NMI handlers
>   if another cpu already panicked
> 
> V2:
> - Use atomic_cmpxchg() instead of current spin_trylock() to exclude
>   concurrent accesses to panic() and crash_kexec()
> - Don't introduce no-lock version of panic() and crash_kexec()
> 
> ---
> 
> Hidehiro Kawai (4):
>       panic/x86: Fix re-entrance problem due to panic on NMI
>       panic/x86: Allow cpus to save registers even if they are looping in NMI context
>       kexec: Fix race between panic() and crash_kexec() called directly
>       x86/apic: Introduce noextnmi boot option
> 
> 
>  Documentation/kernel-parameters.txt |    4 ++++
>  arch/x86/kernel/apic/apic.c         |   17 ++++++++++++++++-
>  arch/x86/kernel/nmi.c               |   15 +++++++++++----
>  arch/x86/kernel/reboot.c            |   11 +++++++++++
>  include/linux/kernel.h              |   21 +++++++++++++++++++++
>  kernel/kexec.c                      |   20 ++++++++++++++++++++
>  kernel/panic.c                      |   23 ++++++++++++++++++++---
>  kernel/watchdog.c                   |    5 +++--
>  8 files changed, 106 insertions(+), 10 deletions(-)
> 
> 
> -- 
> Hidehiro Kawai
> Hitachi, Ltd. Research & Development Group
> 
> 

-- 
Michal Hocko
SUSE Labs
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