On Mon, 9 Sep 2013 14:13:31 +0200
Frederic Weisbecker <fweis...@gmail.com> wrote:
> > In any case the preempt_disable/enable pair there is just plain wrong as
> > Eric pointed out.
>
> Check this:
>
> 34240697d619c439c55f21989680024dcb604aab "rcu: Disable preemption in
> rcu_is_cpu_idle()"
Ug, and that patch does nothing to fix the bug that it reported!
1. Task A on CPU 1 enters rcu_is_cpu_idle() and picks up the
pointer to CPU 1's per-CPU variables.
2. Task B preempts Task A and starts running on CPU 1.
Let's say that B preempts Task A here:
preempt_disable();
ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
preempt_enable();
<preempt>
return ret;
3. Task A migrates to CPU 2.
4. Task B blocks, leaving CPU 1 idle.
5. Task A continues execution on CPU 2, accessing CPU 1's
dyntick-idle information using the pointer fetched in step 1 above,
and finds that CPU 1 is idle.
Yeah, and Task A is using the "ret" from CPU 1!
6. Task A therefore incorrectly concludes that it is executing in
an extended quiescent state, possibly issuing a spurious splat.
Therefore, this commit disables preemption within the
rcu_is_cpu_idle() function.
Where this commit is totally bogus. Sorry, but it is.
This just proves that the caller of rcu_is_cpu_idle() must disable
preemption itself for the entire time that it needs to use the result
of rcu_is_cpu_idle().
-- Steve
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