Re: [PATCH] x86/tsc: use real seqcount_latch in cyc2ns_read_begin()

[kbuild test robot]

Hi Eric,

I love your patch! Perhaps something to improve:

[auto build test WARNING on tip/x86/core]
[also build test WARNING on v4.19-rc7 next-20181011]
[if your patch is applied to the wrong git tree, please drop us a note to help 
improve the system]

url:    
https://github.com/0day-ci/linux/commits/Eric-Dumazet/x86-tsc-use-real-seqcount_latch-in-cyc2ns_read_begin/20181012-065625
config: x86_64-randconfig-x000-201840 (attached as .config)
compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All warnings (new ones prefixed by >>):

   arch/x86/kernel/tsc.c: In function 'cyc2ns_read_begin':
>> arch/x86/kernel/tsc.c:69:33: warning: passing argument 1 of 
>> 'raw_read_seqcount_latch' discards 'const' qualifier from pointer target 
>> type [-Wdiscarded-qualifiers]
      seq = raw_read_seqcount_latch(&c2n->seq);
                                    ^
   In file included from include/linux/time.h:6:0,
                    from include/linux/ktime.h:24,
                    from include/linux/timer.h:6,
                    from include/linux/workqueue.h:9,
                    from include/linux/rhashtable-types.h:15,
                    from include/linux/ipc.h:7,
                    from include/uapi/linux/sem.h:5,
                    from include/linux/sem.h:5,
                    from include/linux/sched.h:15,
                    from arch/x86/kernel/tsc.c:4:
   include/linux/seqlock.h:279:19: note: expected 'seqcount_t * {aka struct 
seqcount *}' but argument is of type 'const seqcount_t * {aka const struct 
seqcount *}'
    static inline int raw_read_seqcount_latch(seqcount_t *s)
                      ^~~~~~~~~~~~~~~~~~~~~~~

vim +69 arch/x86/kernel/tsc.c

    59  
    60  void cyc2ns_read_begin(struct cyc2ns_data *data)
    61  {
    62          const struct cyc2ns *c2n;
    63          int seq;
    64  
    65          preempt_disable_notrace();
    66  
    67          c2n = this_cpu_ptr(&cyc2ns);
    68          do {
  > 69                  seq = raw_read_seqcount_latch(&c2n->seq);
    70                  *data = c2n->data[seq & 1];
    71          } while (read_seqcount_retry(&c2n->seq, seq));
    72  }
    73  

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