Messages in this thread | | | Date | Sun, 10 Jun 2012 11:06:59 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: Allow calls to rcu_exit_user_irq from nesting irqs |
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On Sun, Jun 10, 2012 at 12:55:54AM +0200, Frederic Weisbecker wrote: > On Tue, Jun 05, 2012 at 04:46:40PM -0700, Paul E. McKenney wrote: > > Here you go: > > > > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git rcu/idle > > So I've rebased my nohz cpusets patchset and applied these patches. > During testing I found a bug and realized I need to make rcu_exit_user_irq() > callable from any irq nesting level.
OK. ;-)
> Here is a fix: > > --- > >From c30610d5ed2c292a87f7e32216c3419cdc12dff0 Mon Sep 17 00:00:00 2001 > From: Frederic Weisbecker <fweisbec@gmail.com> > Date: Sat, 9 Jun 2012 14:06:30 +0200 > Subject: [PATCH] rcu: Allow calls to rcu_exit_user_irq from nesting irqs > > rcu_exit_user_irq() which exits RCU idle mode after the current > irq returns has been designed to be called from non nesting irqs > only. > > However the IPI that restarts the tick and exits RCU user-idle mode > in nohz cpusets can happen anytime. For example it can be a nesting > irq by interrupting a softirq. In this case the stack of RCU API > calls becomes: > > ==> IRQ > rcu_irq_enter() > .... > do_softirq { > ===== > IRQ (restart tick IPI) > rcu_irq_enter() > rcu_exit_user_irq() > rcu_irq_exit() > <===== > } > rcu_irq_exit(); > > Hence we need to make rcu_exit_user_irq() callable from any nesting > level of interrupt. > > rcu_enter_user_irq() is only called from non nesting irqs though. But > to stay consistant with the new change we also allow it to be called > from any irq nesting level. > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> > --- > kernel/rcutree.c | 36 +++++++++++++++--------------------- > 1 files changed, 15 insertions(+), 21 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index 1b0dca2..3e84c4c 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -465,11 +465,11 @@ void rcu_user_enter(void) > > /** > * rcu_user_enter_irq - inform RCU that we are going to resume userspace > - * after the current irq returns. > + * after the current non-nesting irq returns. > * > - * This is similar to rcu_user_enter() but in the context of a non > - * nesting irq. After this call, RCU enters into idle mode when the > - * interrupt returns. > + * This is similar to rcu_user_enter() but in the context of an > + * irq. After this call, RCU enters into idle mode when the > + * current non-nesting interrupt returns. > */ > void rcu_user_enter_irq(void) > { > @@ -478,12 +478,9 @@ void rcu_user_enter_irq(void) > > local_irq_save(flags); > rdtp = &__get_cpu_var(rcu_dynticks); > - /* > - * Ensure this irq is a non nesting one interrupting > - * a non-idle RCU state. > - */ > - WARN_ON_ONCE(rdtp->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE + 1); > - rdtp->dynticks_nesting = 1; > + /* Ensure we are interrupting a non-idle RCU state */ > + WARN_ON_ONCE(!(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK)); > + rdtp->dynticks_nesting -= DYNTICK_TASK_EXIT_IDLE;
This will be broken on architectures that can fail to return from interrupts and exceptions and vice versa. The resulting value of rdtp->dynticks_nesting might well go negative, or might fail to reach zero when the outermost interrupt returns.
One workaround would be to add up the relevant fields of preempt_count() and assign the result to rdtp->dynticks_nesting.
> local_irq_restore(flags); > } > > @@ -619,12 +616,12 @@ void rcu_user_exit(void) > > /** > * rcu_user_exit_irq - inform RCU that we won't resume to userspace > - * idle mode after the current irq returns. > + * idle mode after the current non-nesting irq returns. > * > - * This is similar to rcu_user_exit() but in the context of a non > - * nesting irq. This is called when the irq has interrupted a userspace > - * RCU idle mode context. When the interrupt returns after this call, > - * the CPU won't restore the RCU idle mode. > + * This is similar to rcu_user_exit() but in the context of an > + * irq. This is called when the irq has interrupted a userspace > + * RCU idle mode context. When the current non-nesting interrupt > + * returns after this call, the CPU won't restore the RCU idle mode. > */ > void rcu_user_exit_irq(void) > { > @@ -633,12 +630,9 @@ void rcu_user_exit_irq(void) > > local_irq_save(flags); > rdtp = &__get_cpu_var(rcu_dynticks); > - /* > - * Ensure this irq is a non-nesting one interrupting > - * an RCU idle mode. > - */ > - WARN_ON_ONCE(rdtp->dynticks_nesting != 1); > - rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE + 1; > + /* Ensure we are interrupting an RCU idle mode. */ > + WARN_ON_ONCE(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK); > + rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
This one works because all of the interrupt misnesting events that I know of happen from system-call context, not from idle or from user-mode execution.
Thanx, Paul
> local_irq_restore(flags); > } > > -- > 1.7.0.4 > >
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