[V2 PATCH 1/3] x86/panic: Fix re-entrance problem due to panic on NMI

Hidehiro Kawai
2015-07-27T05:11:28Z

If panic on NMI happens just after panic() on the same CPU, panic()
is recursively called. As the result, it stalls after failing to
acquire panic_lock.

To avoid this problem, don't call panic() in NMI context if
we've already entered panic().

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

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>
---
arch/x86/kernel/nmi.c | 15 +++++++++++----
include/linux/kernel.h | 1 +
kernel/panic.c | 13 ++++++++++---
3 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index d05bd2e..5b32d81 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -230,7 +230,8 @@ void unregister_nmi_handler(unsigned int type, const char *name)
}
#endif

- if (panic_on_unrecovered_nmi)
+ if (panic_on_unrecovered_nmi &&
+ atomic_cmpxchg(&panicking_cpu, -1, raw_smp_processor_id()) == -1)
panic("NMI: Not continuing");

pr_emerg("Dazed and confused, but trying to continue\n");
@@ -255,8 +256,13 @@ void unregister_nmi_handler(unsigned int type, const char *name)
reason, smp_processor_id());
show_regs(regs);

- if (panic_on_io_nmi)
- panic("NMI IOCK error: Not continuing");
+ if (panic_on_io_nmi) {
+ if (atomic_cmpxchg(&panicking_cpu, -1, raw_smp_processor_id())
+ == -1)
+ panic("NMI IOCK error: Not continuing");
+ else
+ return; /* We don't want to wait and re-enable NMI */
+ }

/* Re-enable the IOCK line, wait for a few seconds */
reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;
@@ -296,7 +302,8 @@ void unregister_nmi_handler(unsigned int type, const char *name)
reason, smp_processor_id());

pr_emerg("Do you have a strange power saving mode enabled?\n");
- if (unknown_nmi_panic || panic_on_unrecovered_nmi)
+ if ((unknown_nmi_panic || panic_on_unrecovered_nmi) &&
+ atomic_cmpxchg(&panicking_cpu, -1, raw_smp_processor_id()) == -1)
panic("NMI: Not continuing");

pr_emerg("Dazed and confused, but trying to continue\n");
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 5582410..8ca199b 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -442,6 +442,7 @@ extern __scanf(2, 0)
extern int sysctl_panic_on_stackoverflow;

extern bool crash_kexec_post_notifiers;
+extern atomic_t panicking_cpu;

/*
* Only to be used by arch init code. If the user over-wrote the default
diff --git a/kernel/panic.c b/kernel/panic.c
index 04e91ff..7e6b568 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -60,6 +60,8 @@ void __weak panic_smp_self_stop(void)
cpu_relax();
}

+atomic_t panicking_cpu = ATOMIC_INIT(-1);
+
/**
* panic - halt the system
* @fmt: The text string to print
@@ -70,17 +72,17 @@ void __weak panic_smp_self_stop(void)
*/
void panic(const char *fmt, ...)
{
- static DEFINE_SPINLOCK(panic_lock);
static char buf[1024];
va_list args;
long i, i_next = 0;
int state = 0;
+ int old_cpu, this_cpu;

/*
* Disable local interrupts. This will prevent panic_smp_self_stop
* from deadlocking the first cpu that invokes the panic, since
* there is nothing to prevent an interrupt handler (that runs
- * after the panic_lock is acquired) from invoking panic again.
+ * after setting panicking_cpu) from invoking panic again.
*/
local_irq_disable();

@@ -93,8 +95,13 @@ void panic(const char *fmt, ...)
* 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().
+ *
+ * `old_cpu == -1' means we are the first comer.
+ * `old_cpu == this_cpu' means we came here due to panic on NMI.
*/
- if (!spin_trylock(&panic_lock))
+ this_cpu = raw_smp_processor_id();
+ old_cpu = atomic_cmpxchg(&panicking_cpu, -1, this_cpu);
+ if (old_cpu != -1 && old_cpu != this_cpu)
panic_smp_self_stop();

console_verbose();