Commit 93e13a36 authored Jan 12, 2012 by Michael Holzheu Committed by Linus Torvalds Jan 12, 2012

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>

parent bec013c4

kernel/panic.c

+17 −1

Original line number	Diff line number	Diff line
		@@ -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);

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