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



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