kdump: fix crash_kexec()/smp_send_stop() race in panic()

When two CPUs call panic at the same time there is a possible race
condition that can stop kdump.  The first CPU calls crash_kexec() and the
second CPU calls smp_send_stop() in panic() before crash_kexec() finished
on the first CPU.  So the second CPU stops the first CPU and therefore
kdump fails:

1st CPU:
  panic()->crash_kexec()->mutex_trylock(&kexec_mutex)-> do kdump

2nd CPU:
  panic()->crash_kexec()->kexec_mutex already held by 1st CPU
       ->smp_send_stop()-> stop 1st CPU (stop kdump)

This patch fixes the problem by introducing a spinlock in panic that
allows only one CPU to process crash_kexec() and the subsequent panic
code.

All other CPUs call the weak function panic_smp_self_stop() that stops the
CPU itself.  This function can be overloaded by architecture code.  For
example "tile" can use their lower-power "nap" instruction for that.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 1 deletions

diff --git a/kernel/panic.c b/kernel/panic.c
index 6fd09ed6fd90..5dce5404eeef 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -49,6 +49,15 @@ static long no_blink(int state)
 long (*panic_blink)(int state);
 EXPORT_SYMBOL(panic_blink);
 
+/*
+ * Stop ourself in panic -- architecture code may override this
+ */
+void __weak panic_smp_self_stop(void)
+{
+        while (1)
+                cpu_relax();
+}
+
 /**
  *      panic - halt the system
  *      @fmt: The text string to print
@@ -59,6 +68,7 @@ EXPORT_SYMBOL(panic_blink);
  */
 void panic(const char *fmt, ...)
 {
+        static DEFINE_SPINLOCK(panic_lock);
         static char buf[1024];
         va_list args;
         long i, i_next = 0;
@@ -68,8 +78,14 @@ void panic(const char *fmt, ...)
          * It's possible to come here directly from a panic-assertion and
          * not have preempt disabled. Some functions called from here want
          * preempt to be disabled. No point enabling it later though...
+         *
+         * 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().
          */
-        preempt_disable();
+        if (!spin_trylock(&panic_lock))
+                panic_smp_self_stop();
 
         console_verbose();
         bust_spinlocks(1);