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

[河合英宏 / KAWAI，HIDEHIRO]

> 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
[snip]
>    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);
>       ^

I changed to use atomic_xchg() instead of atomic_set() in V3
because atomic_set() doesn't mean memory barrier.  However,
I thought again and there is no need of barrier; there is no
problem if a competitor sees old value of panic_cpu or new one.
So, atomic_set() is sufficient and using it will remove this warning.

I will resend the fixed version later.

> 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);
> 
> ---
> 0-DAY kernel test infrastructure                Open Source Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation


Hidehiro Kawai
Hitachi, Ltd. Research & Development Group