Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:

...

Which is a problem how?

So here is a possible scenario, CPU 0 reads a kcpustat value, and CPU 1 writes it at the same time:

//Initial value of "cpustat" is 0xffffffff
     == CPU 0 ==           == CPU 1 ==

   //load low part
   mov %eax, [cpustat]
                         inc [cpustat]
                         //Update the high part if necessary
                         jnc 1f
                         inc [cpustat + 4]
                         1:
   //load high part
   mov %edx, [cpustat + 4]

Afterward, CPU 0 will think the value is 0x1ffffffff while it's actually 0x100000000.

atomic64_read() and atomic64_set() are supposed to take care of that, without even the need for _inc() or _add() parts that use LOCK.

Sure I get that, but again, why is that a problem,.. who relies on these statistics that makes it a problem?