Messages in this thread | | | Date | Tue, 18 Feb 2020 19:59:20 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held |
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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@infradead.org> > > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com> > > --- > > 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 > >
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