linux-kernel - Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit grace-period counter

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Message-ID: <20201123195543.GC1437@paulmck-ThinkPad-P72>
Date:   Mon, 23 Nov 2020 11:55:43 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        kent.overstreet@...il.com
Subject: Re: [PATCH v2 tip/core/rcu 1/6] srcu: Make Tiny SRCU use multi-bit
 grace-period counter

On Mon, Nov 23, 2020 at 10:01:13AM +0530, Neeraj Upadhyay wrote:
> On 11/21/2020 6:29 AM, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.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@...il.com>
> > [ paulmck: Fix __srcu_read_lock() idx computation Neeraj per Upadhyay. ]
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.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..d9edb67 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) + 1) & 0x2) >> 1;
> >   	WRITE_ONCE(ssp->srcu_lock_nesting[idx], ssp->srcu_lock_nesting[idx] + 1);
> >   	return idx;
> >   }
> 
> Need change in idx calcultion in srcu_torture_stats_print() ?
> 
> static inline void srcu_torture_stats_print(struct srcu_struct *ssp,
>   idx = READ_ONCE(ssp->srcu_idx) & 0x1;

Excellent point!  It should match the calculation in __srcu_read_lock(),
shouldn't it?  I have updated this, thank you!

							Thanx, Paul

> Thanks
> Neeraj
> 
> > 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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