On Mon, Sep 09, 2013 at 08:39:26AM -0400, Steven Rostedt wrote: > 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().
Sorry, I don't understand your point here. What's wrong with checking the ret from another CPU? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
