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

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