Messages in this thread | | | Date | Wed, 5 Oct 2011 18:59:24 -0700 | From | "Paul E. McKenney" <> | Subject | Re: linux-next-20110923: warning kernel/rcutree.c:1833 |
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On Wed, Oct 05, 2011 at 05:58:58PM -0700, Paul E. McKenney wrote: > On Mon, Oct 03, 2011 at 09:30:36AM -0700, Paul E. McKenney wrote: > > On Mon, Oct 03, 2011 at 03:03:48PM +0200, Frederic Weisbecker wrote: > > > On Sun, Oct 02, 2011 at 05:32:47PM -0700, Paul E. McKenney wrote: > > > > > > -void rcu_irq_enter(void) > > > > > > +int rcu_is_cpu_idle(void) > > > > > > { > > > > > > - rcu_exit_nohz(); > > > > > > + return (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0; > > > > > > } > > > > > > > > > > So that's not used in this patch but it's interesting for me > > > > > to backport "rcu: Detect illegal rcu dereference in extended quiescent state". > > > > > > > > Yep, that is why it is there. > > > > > > Ok. > > > > > > > > > > > > The above should be read from a preempt disabled section though > > > > > (remember "rcu: Fix preempt-unsafe debug check of rcu extended quiescent state") > > > > > > > > Yes, and that is why the last line of the header comment reads "The > > > > caller must have at least disabled preemption." Disabling preemption > > > > is not necessary in Tiny RCU because there is no other CPU for the task > > > > to go to. (Right?) > > > > > > Right. > > > > > > > > Those functions should probably lay in a separate patch. But I don't mind > > > > > much keeping the things as is and use these APIs in my next patches though. > > > > > I'll just fix the preempt enabled thing above. > > > > > > > > Or were you saying that you wish to make calls to rcu_is_cpu_idle() > > > > that have preemption enabled? > > > > > > Yeah. That's going to be called from places like rcu_read_lock_held() > > > and things like this that don't need to disable preemption themselves. > > > > > > Would be better to disable preemption from that function. > > > > Hmmm... This might be a good use for the "drive-by" per-CPU access > > functions. > > > > No, that doesn't work. We could pick up the pointer, switch to another > > CPU, the original CPU could run a task that blocks before we start running, > > and then we could incorrectly decide that we were running in idle context, > > issuing a spurious warning. This approach would only work in environments > > that (unlike the Linux kernel) mapped all the per-CPU variables to the > > same virtual address on all CPUs. (DYNIX/ptx did this, but this leads > > to other problems, like being unable to reasonably access other CPUs' > > variables. Double mapping has other issues on some architectures.) > > > > OK, agreed. I will make this function disable preemption. > > > > > > And I can split the patch easily enough while keeping the diff the same, > > > > so you should be able to do your porting on top of the existing code. > > > > > > No I'm actually pretty fine with the current state. Whether that's defined > > > in this patch or a following one is actually not important. > > > > Fair enough! > > And here is an update that might handle an irq entry/exit miscounting > problem. Thanks to Arjan van de Ven for pointing out that my earlier > approach would in fact miscount irq entries/exits in face of things like > upcalls to user-mode helpers. > > This is experimental, and might well hurt more than it helps. Testing > ongoing. Applies on top of my "Track idleness independent of idle tasks" > commit. Right... And the tracing relies on a later patch, so feel free > to yank the calls to trace_rcu_dyntick() on the off-chance that you are > crazy enough to actually try this. > > Thoughts?
For the code currently in mainline, I hasten to add. For your use, Frederic, I need to handle the case where a user process is idle from an RCU viewpoint. I will be looking into this, but in the meantime I wanted to prove/disprove that this is the source of the failures that I have been seeing.
Thanx, Paul
> ------------------------------------------------------------------------ > > Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index 06c0ed4..d4247e0 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -356,6 +356,11 @@ void rcu_idle_enter(void) > > local_irq_save(flags); > rdtp = &__get_cpu_var(rcu_dynticks); > + if (!idle_cpu(smp_processor_id())) { > + trace_rcu_dyntick("--|", rdtp->dynticks_nesting); > + local_irq_restore(flags); > + return; > + } > if (--rdtp->dynticks_nesting) { > trace_rcu_dyntick("--=", rdtp->dynticks_nesting); > local_irq_restore(flags); > @@ -384,6 +389,11 @@ void rcu_idle_exit(void) > > local_irq_save(flags); > rdtp = &__get_cpu_var(rcu_dynticks); > + if (!idle_cpu(smp_processor_id())) { > + trace_rcu_dyntick("++|", rdtp->dynticks_nesting); > + local_irq_restore(flags); > + return; > + } > if (rdtp->dynticks_nesting++) { > trace_rcu_dyntick("++=", rdtp->dynticks_nesting); > local_irq_restore(flags);
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