Re: [PATCH RFC tip/core/rcu 3/5] srcu: Provide internal interface to start a Tree SRCU grace period

[Paul E. McKenney]

On Fri, Nov 20, 2020 at 05:06:50PM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 11/17/2020 6:10 AM, paulmck@xxxxxxxxxx wrote:
> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > 
> > There is a need for a polling interface for SRCU grace periods.
> > This polling needs to initiate an SRCU grace period without having
> > to queue (and manage) a callback.  This commit therefore splits the
> > Tree SRCU __call_srcu() function into callback-initialization and
> > queuing/start-grace-period portions, with the latter in a new function
> > named srcu_gp_start_if_needed().  This function may be passed a NULL
> > callback pointer, in which case it will refrain from queuing anything.
> > 
> > Why have the new function mess with queuing?  Locking considerations,
> > of course!
> > 
> > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@xxxxxxxxxxxxxx/
> > Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> 
> Reviewed-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>

I applied both Reviewed-bys, thank you!

							Thanx, Paul

> Thanks
> Neeraj
> 
> >   kernel/rcu/srcutree.c | 66 +++++++++++++++++++++++++++++----------------------
> >   1 file changed, 37 insertions(+), 29 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 79b7081..d930ece 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -808,6 +808,42 @@ static void srcu_leak_callback(struct rcu_head *rhp)
> >   }
> >   /*
> > + * Start an SRCU grace period, and also queue the callback if non-NULL.
> > + */
> > +static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rhp, bool do_norm)
> > +{
> > +	unsigned long flags;
> > +	int idx;
> > +	bool needexp = false;
> > +	bool needgp = false;
> > +	unsigned long s;
> > +	struct srcu_data *sdp;
> > +
> > +	idx = srcu_read_lock(ssp);
> > +	sdp = raw_cpu_ptr(ssp->sda);
> > +	spin_lock_irqsave_rcu_node(sdp, flags);
> > +	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> > +	rcu_segcblist_advance(&sdp->srcu_cblist,
> > +			      rcu_seq_current(&ssp->srcu_gp_seq));
> > +	s = rcu_seq_snap(&ssp->srcu_gp_seq);
> > +	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
> > +	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> > +		sdp->srcu_gp_seq_needed = s;
> > +		needgp = true;
> > +	}
> > +	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
> > +		sdp->srcu_gp_seq_needed_exp = s;
> > +		needexp = true;
> > +	}
> > +	spin_unlock_irqrestore_rcu_node(sdp, flags);
> > +	if (needgp)
> > +		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> > +	else if (needexp)
> > +		srcu_funnel_exp_start(ssp, sdp->mynode, s);
> > +	srcu_read_unlock(ssp, idx);
> > +}
> > +
> > +/*
> >    * Enqueue an SRCU callback on the srcu_data structure associated with
> >    * the current CPU and the specified srcu_struct structure, initiating
> >    * grace-period processing if it is not already running.
> > @@ -838,13 +874,6 @@ static void srcu_leak_callback(struct rcu_head *rhp)
> >   static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >   			rcu_callback_t func, bool do_norm)
> >   {
> > -	unsigned long flags;
> > -	int idx;
> > -	bool needexp = false;
> > -	bool needgp = false;
> > -	unsigned long s;
> > -	struct srcu_data *sdp;
> > -
> >   	check_init_srcu_struct(ssp);
> >   	if (debug_rcu_head_queue(rhp)) {
> >   		/* Probable double call_srcu(), so leak the callback. */
> > @@ -853,28 +882,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >   		return;
> >   	}
> >   	rhp->func = func;
> > -	idx = srcu_read_lock(ssp);
> > -	sdp = raw_cpu_ptr(ssp->sda);
> > -	spin_lock_irqsave_rcu_node(sdp, flags);
> > -	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> > -	rcu_segcblist_advance(&sdp->srcu_cblist,
> > -			      rcu_seq_current(&ssp->srcu_gp_seq));
> > -	s = rcu_seq_snap(&ssp->srcu_gp_seq);
> > -	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
> > -	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> > -		sdp->srcu_gp_seq_needed = s;
> > -		needgp = true;
> > -	}
> > -	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
> > -		sdp->srcu_gp_seq_needed_exp = s;
> > -		needexp = true;
> > -	}
> > -	spin_unlock_irqrestore_rcu_node(sdp, flags);
> > -	if (needgp)
> > -		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> > -	else if (needexp)
> > -		srcu_funnel_exp_start(ssp, sdp->mynode, s);
> > -	srcu_read_unlock(ssp, idx);
> > +	srcu_gp_start_if_needed(ssp, rhp, do_norm);
> >   }
> >   /**
> > 
> 
> -- 
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