Re: [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held

[Joel Fernandes]

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?

thanks,

 - Joel


>  			raise_softirq_irqoff(RCU_SOFTIRQ);
>  		} else {
>  			// Enabling BH or preempt does reschedule, so...
> -- 
> 2.20.1
>