Re: [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting

["Paul E. McKenney" <paulmck@kernel.org>]

On Sat, Nov 16, 2019 at 09:04:56PM +0800, Lai Jiangshan wrote:
> On 2019/11/1 8:33 下午, Paul E. McKenney wrote:
> > On Thu, Oct 31, 2019 at 10:08:03AM +0000, Lai Jiangshan wrote:
> > > Negative ->rcu_read_lock_nesting was introduced to prevent
> > > scheduler deadlock which was just prevented by deferred qs.
> > > So negative ->rcu_read_lock_nesting is useless now and
> > > rcu_read_unlock() can be simplified.
> > > 
> > > And negative ->rcu_read_lock_nesting is bug-prone,
> > > it is good to kill it.
> > > 
> > > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> > > ---
> > >   kernel/rcu/tree_exp.h    | 30 ++----------------------------
> > >   kernel/rcu/tree_plugin.h | 21 +++++----------------
> > >   2 files changed, 7 insertions(+), 44 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > > index c0d06bce35ea..9dcbd2734620 100644
> > > --- a/kernel/rcu/tree_exp.h
> > > +++ b/kernel/rcu/tree_exp.h
> > > @@ -621,11 +621,11 @@ static void rcu_exp_handler(void *unused)
> > >   	 * report the quiescent state, otherwise defer.
> > >   	 */
> > >   	if (!t->rcu_read_lock_nesting) {
> > > +		rdp->exp_deferred_qs = true;
> > >   		if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
> > >   		    rcu_dynticks_curr_cpu_in_eqs()) {
> > > -			rcu_report_exp_rdp(rdp);
> > > +			rcu_preempt_deferred_qs(t);
> > >   		} else {
> > > -			rdp->exp_deferred_qs = true;
> > >   			set_tsk_need_resched(t);
> > >   			set_preempt_need_resched();
> > >   		}
> > > @@ -646,32 +646,6 @@ static void rcu_exp_handler(void *unused)
> > >   		WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > >   		return;
> > >   	}
> > > -
> > > -	/*
> > > -	 * The final and least likely case is where the interrupted
> > > -	 * code was just about to or just finished exiting the RCU-preempt
> > > -	 * read-side critical section, and no, we can't tell which.
> > > -	 * So either way, set ->deferred_qs to flag later code that
> > > -	 * a quiescent state is required.
> > > -	 *
> > > -	 * If the CPU is fully enabled (or if some buggy RCU-preempt
> > > -	 * read-side critical section is being used from idle), just
> > > -	 * invoke rcu_preempt_deferred_qs() to immediately report the
> > > -	 * quiescent state.  We cannot use rcu_read_unlock_special()
> > > -	 * because we are in an interrupt handler, which will cause that
> > > -	 * function to take an early exit without doing anything.
> > > -	 *
> > > -	 * Otherwise, force a context switch after the CPU enables everything.
> > > -	 */
> > > -	rdp->exp_deferred_qs = true;
> > > -	if (rcu_preempt_need_deferred_qs(t) &&
> > > -	    (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
> > > -	    WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()))) {
> > > -		rcu_preempt_deferred_qs(t);
> > > -	} else {
> > > -		set_tsk_need_resched(t);
> > > -		set_preempt_need_resched();
> > > -	}
> > >   }
> > >   /* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index dbded2b8c792..c62631c79463 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -344,8 +344,6 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
> > >   }
> > >   /* Bias and limit values for ->rcu_read_lock_nesting. */
> > > -#define RCU_NEST_BIAS INT_MAX
> > > -#define RCU_NEST_NMAX (-INT_MAX / 2)
> > >   #define RCU_NEST_PMAX (INT_MAX / 2)
> > >   /*
> > > @@ -373,21 +371,15 @@ void __rcu_read_unlock(void)
> > >   {
> > >   	struct task_struct *t = current;
> > > -	if (t->rcu_read_lock_nesting != 1) {
> > > -		--t->rcu_read_lock_nesting;
> > > -	} else {
> > > +	if (--t->rcu_read_lock_nesting == 0) {
> > >   		barrier();  /* critical section before exit code. */
> > > -		t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
> > > -		barrier();  /* assign before ->rcu_read_unlock_special load */
> > 
> > But if we take an interrupt here, and the interrupt handler contains
> > an RCU read-side critical section, don't we end up in the same hole
> > that resulted in this article when the corresponding rcu_read_unlock()
> > executes?  https://lwn.net/Articles/453002/
> 
> Hello, Paul
> 
> I'm replying the email of V1, which is relying on deferred_qs changes
> in [PATCH 07/11] (V1).
> ([PATCH 04/11](V1) relies on it too as you pointed out)
> 
> I hope I can answer the question wrt https://lwn.net/Articles/453002/
> maybe partially.
> 
> With the help of deferred_qs mechanism and the special.b.deferred_qs
> bit, I HOPED rcu_read_unlock_special() can find if itself is
> risking in scheduler locks via special.b.deferred_qs bit.
> 
> --t->rcu_read_lock_nesting;
> //outmost rcu c.s, rcu_read_lock_nesting is 0. but special is not zero
> INTERRUPT
>  // the fallowing code will normally be in_interrupt()
>  // or NOT in_interrupt() when wakeup_softirqd() in invoke_softirq()
>  // or NOT in_interrupt() when preempt_shedule_irq()
>  // or other cases I missed.
>  scheduler_lock()
>  rcu_read_lock()
>  rcu_read_unlock()
>   // special has been set but with no special.b.deferred_qs
>   rcu_read_unlock_special()
>    raise_softirq_irqoff()
>     wake_up() when !in_interrupt() // dead lock
> 
> preempt_shedule_irq() is guaranteed to clear rcu_read_unlock_special
> when rcu_read_lock_nesting = 0 before calling into scheduler locks.
> 
> But, at least, what caused my hope to be failed was the case
> wakeup_softirqd() in invoke_softirq() (which was once protected by
> softirq in about 2 years between ec433f0c5152 and facd8b80c67a).
> I don't think it is hard to fix it if we keep using
> special.b.deferred_qs as this V1 series.

It is quite possible that special.b.deferred_qs might be useful
for debugging.  But it should now be possible to take care of the
nohz_full issue for expedited grace periods, which might in turn allow
rcu_read_unlock_special() to avoid acquiring scheduler locks.

This could avoid the need for negative ->rcu_read_lock_nesting,
in turn allowing your simplified _rcu_read_unlock().

Would you like to do the expedited grace-period modifications, or
would you rather that I do so?

						Thanx, Paul