Re: [PATCH RFC tip/core/rcu 5/5] srcu: Provide polling interfaces for Tree SRCU grace periods

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

On Fri, Nov 20, 2020 at 05:31:43PM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 11/17/2020 6:10 AM, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > There is a need for a polling interface for SRCU grace
> > periods, so this commit supplies get_state_synchronize_srcu(),
> > start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
> > purpose.  The first can be used if future grace periods are inevitable
> > (perhaps due to a later call_srcu() invocation), the second if future
> > grace periods might not otherwise happen, and the third to check if a
> > grace period has elapsed since the corresponding call to either of the
> > first two.
> > 
> > As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
> > the return value from either get_state_synchronize_srcu() or
> > start_poll_synchronize_srcu() must be passed in to a later call to
> > poll_state_synchronize_srcu().
> > 
> > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> > Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
> > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >   kernel/rcu/srcutree.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++---
> >   1 file changed, 60 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index d930ece..015d80e 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -810,7 +810,8 @@ 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)
> > +static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> > +					     struct rcu_head *rhp, bool do_norm)
> >   {
> >   	unsigned long flags;
> >   	int idx;
> > @@ -822,7 +823,8 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
> >   	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);
> > +	if (rhp)
> > +		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);
> > @@ -841,6 +843,7 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
> >   	else if (needexp)
> >   		srcu_funnel_exp_start(ssp, sdp->mynode, s);
> >   	srcu_read_unlock(ssp, idx);
> > +	return s;
> >   }
> >   /*
> > @@ -882,7 +885,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> >   		return;
> >   	}
> >   	rhp->func = func;
> > -	srcu_gp_start_if_needed(ssp, rhp, do_norm);
> > +	(void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
> >   }
> >   /**
> > @@ -1011,6 +1014,60 @@ void synchronize_srcu(struct srcu_struct *ssp)
> >   }
> >   EXPORT_SYMBOL_GPL(synchronize_srcu);
> > +/**
> > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> > + * @ssp: srcu_struct to provide cookie for.
> > + *
> > + * This function returns a cookie that can be passed to
> > + * poll_state_synchronize_srcu(), which will return true if a full grace
> > + * period has elapsed in the meantime.  It is the caller's responsibility
> > + * to make sure that grace period happens, for example, by invoking
> > + * call_srcu() after return from get_state_synchronize_srcu().
> > + */
> > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > +	// Any prior manipulation of SRCU-protected data must happen
> > +        // before the load from ->srcu_gp_seq.
> > +	smp_mb();
> > +	return rcu_seq_snap(&ssp->srcu_gp_seq);
> > +}
> > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> > +
> > +/**
> > + * start_poll_synchronize_srcu - Provide cookie and start grace period
> > + * @ssp: srcu_struct to provide cookie for.
> > + *
> > + * This function returns a cookie that can be passed to
> > + * poll_state_synchronize_srcu(), which will return true if a full grace
> > + * period has elapsed in the meantime.  Unlike get_state_synchronize_srcu(),
> > + * this function also ensures that any needed SRCU grace period will be
> > + * started.  This convenience does come at a cost in terms of CPU overhead.
> > + */
> > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> > +{
> > +	return srcu_gp_start_if_needed(ssp, NULL, true);
> > +}
> > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> > +
> > +/**
> > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> > + * @ssp: srcu_struct to provide cookie for.
> > + * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
> > + *
> > + * This function takes the cookie that was returned from either
> > + * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
> > + * returns @true if an SRCU grace period elapsed since the time that the
> > + * cookie was created.
> > + */
> > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> > +{
> > +	if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
> > +		return false;
> > +	smp_mb(); // ^^^
> 
> Minor: Should this comment be more descriptive?

You have to read between the lines?  ;-)

How about like this?

							Thanx, Paul

------------------------------------------------------------------------

bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
{
	if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
		return false;
	// Ensure that the end of the SRCU grace period happens before
	// any subsequent code that the caller might execute.
	smp_mb(); // ^^^
	return true;
}