jiffies_64 vs. jiffies

[Atsushi Nemoto <anemo@xxxxxxxxxxxxx>]

Hi.  I noticed that the 'jiffies' variable has 'wall_jiffies + 1'
value in most of time.  I'm using MIPS platform but I think this is
same for other platforms.

I suppose this is due to gcc does not know that jiffies_64 and jiffies
share same place.

In kernel/timer.c:

static inline void update_times(void)
{
	unsigned long ticks;

	ticks = jiffies - wall_jiffies;
	if (ticks) {
		wall_jiffies += ticks;
		update_wall_time(ticks);
	}
	calc_load(ticks);
}
  
void do_timer(struct pt_regs *regs)
{
	jiffies_64++;
	update_times();
	softlockup_tick(regs);
}

This is compiled MIPS code (gcc 3.4.5):

80056db4 <do_timer>:
...
80056de0:       lui     a3,0x8033
80056de4:       lw      v0,21400(a3)	# load jiffies_64(lo)
80056de8:       lui     a1,0x8033
80056dec:       lui     t1,0x8033
80056df0:       lw      a2,21400(a1)	# load jiffies
80056df4:       lw      v1,21404(a3)	# load jiffies_64(hi)
80056df8:       addiu   v0,v0,1		# inc jiffies_64(lo)
80056dfc:       lw      t0,21360(t1)	# load wall_jiffies
80056e00:       sltiu   a1,v0,1		# calc carry
80056e04:       addu    v1,v1,a1	# add carry to jiffies_64(hi)
80056e08:       subu    s4,a2,t0	# calc ticks (jiffies - wall_jiffies)
80056e0c:       sw      v0,21400(a3)	# store jiffies_64(lo)
80056e10:       sw      v1,21404(a3)	# store jiffies_64(hi)
80056e14:       beqz    s4,80057060 <do_timer+0x2ac>
80056e18:       move    s8,a0

The 'tick' variable is calculated using 'jiffies' value before
incrementing 'jiffies_64'.  As a result, wall_jiffies will always one
smaller then jiffies on elsewhere.

I also checked x86 code (3.4.4).

c012696a <do_timer>:
...
c012696e:       mov    0xc0482400,%eax	# load jiffies
c0126973:       addl   $0x1,0xc0482400	# inc jiffies_64(lo)
c012697a:       mov    0xc041a230,%edx	# load wall_jiffies
c0126980:       mov    %eax,%ebx
c0126982:       adcl   $0x0,0xc0482404	# add carry to jiffies_64(hi)
c0126989:       sub    %edx,%ebx	# calc ticks (jiffies - wall_jiffies)
c012698b:       jne    c01269a0 <do_timer+0x36>

Though I'm not familiar with x86, it looks same.

Is this really expected code?  If no, how it can be fixed?  Insert
"barrier()" right after "jiffies_64++" ?

---
Atsushi Nemoto