Re: [PATCH RFC tip/core/rcu 1/5] srcu: Make Tiny SRCU use multi-bit grace-period counter

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

On Thu, Nov 19, 2020 at 01:44:49PM +0530, Neeraj Upadhyay wrote:
> Hi Paul,
> 
> 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.  This
> > polling needs to distinguish between an SRCU instance being idle on the
> > one hand or in the middle of a grace period on the other.  This commit
> > therefore converts the Tiny SRCU srcu_struct structure's srcu_idx from
> > a defacto boolean to a free-running counter, using the bottom bit to
> > indicate that a grace period is in progress.  The second-from-bottom
> > bit is thus used as the index returned by srcu_read_lock().
> > 
> > Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> > Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >   include/linux/srcutiny.h | 4 ++--
> >   kernel/rcu/srcutiny.c    | 5 +++--
> >   2 files changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > index 5a5a194..fed4a2d 100644
> > --- a/include/linux/srcutiny.h
> > +++ b/include/linux/srcutiny.h
> > @@ -15,7 +15,7 @@
> >   struct srcu_struct {
> >   	short srcu_lock_nesting[2];	/* srcu_read_lock() nesting depth. */
> > -	short srcu_idx;			/* Current reader array element. */
> > +	unsigned short srcu_idx;	/* Current reader array element in bit 0x2. */
> >   	u8 srcu_gp_running;		/* GP workqueue running? */
> >   	u8 srcu_gp_waiting;		/* GP waiting for readers? */
> >   	struct swait_queue_head srcu_wq;
> > @@ -59,7 +59,7 @@ static inline int __srcu_read_lock(struct srcu_struct *ssp)
> >   {
> >   	int idx;
> > -	idx = READ_ONCE(ssp->srcu_idx);
> > +	idx = (READ_ONCE(ssp->srcu_idx) & 0x2) / 2;
> 
> Should we use bit 0x2 of (READ_ONCE(ssp->srcu_idx) + 1) , if GP
> (srcu_drive_gp()) is in progress? Or am I missing something here?
> 
> idx = ((READ_ONCE(ssp->srcu_idx) +1) & 0x2) / 2;

You miss nothing!  I am running about 200 hours of concurrent rcutorture
of the SRCU-t and SRCU-u scenarios, but I must admit that this race could
be hard to hit.  But it could of course result in too-short grace periods.
I will fold this into the original with attribution.

> Also, any reason for using divison instead of shift; something to
> do with 16-bit srcu_idx which I am missing here?

I just figure that the compiler is better at selecting instructions
than I am.  Either would work.

							Thanx, Paul

> Thanks
> Neeraj
> 
> >   	WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1);
> >   	return idx;
> >   }
> > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > index 6208c1d..5598cf6 100644
> > --- a/kernel/rcu/srcutiny.c
> > +++ b/kernel/rcu/srcutiny.c
> > @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *wp)
> >   	ssp->srcu_cb_head = NULL;
> >   	ssp->srcu_cb_tail = &ssp->srcu_cb_head;
> >   	local_irq_enable();
> > -	idx = ssp->srcu_idx;
> > -	WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx);
> > +	idx = (ssp->srcu_idx & 0x2) / 2;
> > +	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
> >   	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
> >   	swait_event_exclusive(ssp->srcu_wq, !READ_ONCE(ssp->srcu_lock_nesting[idx]));
> >   	WRITE_ONCE(ssp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */
> > +	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
> >   	/* Invoke the callbacks we removed above. */
> >   	while (lh) {
> > 
> 
> -- 
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