Re: [V4 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

[kbuild test robot]

Hi Hidehiro,

[auto build test results on v4.3-rc2 -- if it's inappropriate base, please ignore]

config: ia64-allyesconfig (attached as .config)
reproduce:
  wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
  chmod +x ~/bin/make.cross
  git checkout 0077681103150af584e5e592c0238fd010654c26
  # save the attached .config to linux build tree
  make.cross ARCH=ia64 

All warnings (new ones prefixed by >>):

   In file included from arch/ia64/include/uapi/asm/intrinsics.h:21:0,
                    from arch/ia64/include/asm/intrinsics.h:10,
                    from arch/ia64/include/asm/bitops.h:18,
                    from include/linux/bitops.h:36,
                    from include/linux/kernel.h:10,
                    from include/linux/list.h:8,
                    from include/linux/preempt.h:10,
                    from include/linux/spinlock.h:50,
                    from include/linux/mmzone.h:7,
                    from include/linux/gfp.h:5,
                    from include/linux/mm.h:9,
                    from kernel/kexec_core.c:12:
   kernel/kexec_core.c: In function 'crash_kexec':
   arch/ia64/include/uapi/asm/cmpxchg.h:56:2: warning: value computed is not used [-Wunused-value]
    ((__typeof__(*(ptr))) __xchg((unsigned long) (x), (ptr), sizeof(*(ptr))))
     ^
   arch/ia64/include/asm/atomic.h:135:30: note: in expansion of macro 'xchg'
    #define atomic_xchg(v, new) (xchg(&((v)->counter), new))
                                 ^
>> kernel/kexec_core.c:899:3: note: in expansion of macro 'atomic_xchg'
      atomic_xchg(&panic_cpu, -1);
      ^

vim +/atomic_xchg +899 kernel/kexec_core.c

   883	
   884		/*
   885		 * Only one CPU is allowed to execute the crash_kexec() code as with
   886		 * panic().  Otherwise parallel calls of panic() and crash_kexec()
   887		 * may stop each other.  To exclude them, we use panic_cpu here too.
   888		 */
   889		this_cpu = raw_smp_processor_id();
   890		old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
   891		if (old_cpu == -1) {
   892			/* This is the 1st CPU which comes here, so go ahead. */
   893			__crash_kexec(regs);
   894	
   895			/*
   896			 * Reset panic_cpu to allow another panic()/crash_kexec()
   897			 * call.
   898			 */
 > 899			atomic_xchg(&panic_cpu, -1);
   900		}
   901	}
   902	
   903	size_t crash_get_memory_size(void)
   904	{
   905		size_t size = 0;
   906	
   907		mutex_lock(&kexec_mutex);

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