Re: Does cli() need to be called before reading avenrun?

[Robert Love <rml@tech9.net>]

On Wed, 2003-01-01 at 19:39, Peter Benie wrote:
> In kernel 2.4, in sys_sysinfo(), the code reads:
> 
>    cli();
>    val.uptime = jiffies / HZ;
> 
>    val.loads[0] = avenrun[0] << (SI_LOAD_SHIFT - FSHIFT);
>    val.loads[1] = avenrun[1] << (SI_LOAD_SHIFT - FSHIFT);
>    val.loads[2] = avenrun[2] << (SI_LOAD_SHIFT - FSHIFT);
> 
>    val.procs = nr_threads-1;
>    sti();
> 
> In loadavg_read_proc, the code is in essence the same, except that it
> isn't wrapped in cli/sti.  
> 
> Is there a reason for the cli?

Interrupts off protects the reading of avenrun[] - it is only written to
from the timer interrupt, so you can safely read it when interrupts are
disabled.

The reason we need some sort of protection is that there are the three
array entries, so we need to make sure we read all three atomically (not
that its a huge deal if we do not).

So I guess the proc code needs to disable interrupts, too.  I am on my
laptop and only have a 2.5 tree - sure it is not disabled somewhere in
there?

Note in 2.5 we replaced the cli() with a read_lock() on xtime_lock.

	Robert Love