On Tue, Feb 18, 2020 at 10:31:47PM -0500, Joel Fernandes wrote:
> On Sat, Nov 02, 2019 at 12:45:53PM +0000, Lai Jiangshan wrote:
> > Ever since preemption was introduced to linux kernel,
> > irq disabled spinlocks are always held with preemption
> > disabled. One of the reason is that sometimes we need
> > to use spin_unlock() which will do preempt_enable()
> > to unlock the irq disabled spinlock with keeping irq
> > disabled. So preempt_count can be used to test whether
> > scheduler locks is possible held.
> >
> > CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> > kernel/rcu/tree_plugin.h | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 0982e9886103..aba5896d67e3 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -603,10 +603,14 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > tick_nohz_full_cpu(rdp->cpu);
> > // Need to defer quiescent state until everything is enabled.
> > if (irqs_were_disabled && use_softirq &&
> > - (in_interrupt() ||
> > - (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
> > + (in_interrupt() || (exp && !preempt_bh_were_disabled))) {
> > // Using softirq, safe to awaken, and we get
> > // no help from enabling irqs, unlike bh/preempt.
> > + // in_interrupt(): raise_softirq_irqoff() is
> > + // guaranteed not to not do wakeup
> > + // !preempt_bh_were_disabled: scheduler locks cannot
> > + // be held, since spinlocks are always held with
> > + // preempt_disable(), so the wakeup will be safe.
>
> This means if preemption is disabled for any reason (other than scheduler
> locks), such as acquiring an unrelated lock that is not held by the
> scheduler, then the softirq would not be raised even if it was safe to
> do so. From that respect, it seems a step back no?
This patch was one of the things motivating me to turn tick on for
nohz_full CPUs that spend too long in the kernel. Given that change,
this patch can be (and recently was) made more straightforward. Prior to
the nohz_full change, Lai was kind of between a rock and a hard place
on this one. ;-)
Thanx, Paul
> thanks,
>
> - Joel
>
>
> > raise_softirq_irqoff(RCU_SOFTIRQ);
> > } else {
> > // Enabling BH or preempt does reschedule, so...
> > --
> > 2.20.1
> >
