Re: [RFC] Large scale kernel performance recording

[Abramo Bagnara]

Keith Owens wrote:
> 
> Sigh.  Why do you always spot the typos *after* pressing send?
> 
> On Thu, 29 Mar 2001 00:07:02 +1000,
> Keith Owens <[e-mail]-net> wrote:
> >  copy_desired_counter_values()
> >  {
> >    volatile int *p_flag = /* address of flag for desired cpu */;
> >    volatile __s64 *p_counter = /* address of counter for desired cpu and instance */;
> >    __s64 counter; /* local copy */
> >
> >    do {
> >      counter = *p_counter;
> >    } while (!*p_flag && counter == *p_counter);
> >  }
> 
> Should be while (*p_flag || counter != *p_counter);

And here you've the race:

kernel:					user:
++flag;
wmb();
counter += delta; (first 32 bits)
					counter = *p_counter;
counter += delta; (last 32 bits)
wmb();
--flag;
wmb();
...					*p_flag test
++flag;
counter += delta; (first 32 bits)
					counter == *p_counter test
counter += delta; (last 32 bits)
wmb();
--flag;
wmb();

Improbable but theoretically possible.

-- 
Abramo Bagnara                       mailto:[e-mail]

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