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Date:   Sun, 22 Nov 2020 10:01:05 -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
Subject: Re: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces
 for Tiny SRCU grace periods

On Sun, Nov 22, 2020 at 07:57:26PM +0530, Neeraj Upadhyay wrote:
> On 11/21/2020 5:43 AM, Paul E. McKenney wrote:
> > On Fri, Nov 20, 2020 at 05:28:32PM +0530, Neeraj Upadhyay wrote:
> > > Hi Paul,
> > > 
> > > On 11/17/2020 6:10 AM, paulmck@...nel.org wrote:
> > > > From: "Paul E. McKenney" <paulmck@...nel.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@...il.com>
> > > > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > ---
> > > >    include/linux/rcupdate.h |  2 ++
> > > >    include/linux/srcu.h     |  3 +++
> > > >    include/linux/srcutiny.h |  1 +
> > > >    kernel/rcu/srcutiny.c    | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
> > > >    4 files changed, 56 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > index de08264..e09c0d8 100644
> > > > --- a/include/linux/rcupdate.h
> > > > +++ b/include/linux/rcupdate.h
> > > > @@ -33,6 +33,8 @@
> > > >    #define ULONG_CMP_GE(a, b)	(ULONG_MAX / 2 >= (a) - (b))
> > > >    #define ULONG_CMP_LT(a, b)	(ULONG_MAX / 2 < (a) - (b))
> > > >    #define ulong2long(a)		(*(long *)(&(a)))
> > > > +#define USHORT_CMP_GE(a, b)	(USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
> > > > +#define USHORT_CMP_LT(a, b)	(USHRT_MAX / 2 < (unsigned short)((a) - (b)))
> > > >    /* Exported common interfaces */
> > > >    void call_rcu(struct rcu_head *head, rcu_callback_t func);
> > > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > > index e432cc9..a0895bb 100644
> > > > --- a/include/linux/srcu.h
> > > > +++ b/include/linux/srcu.h
> > > > @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
> > > >    int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
> > > >    void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
> > > >    void synchronize_srcu(struct srcu_struct *ssp);
> > > > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
> > > > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
> > > > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
> > > >    #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > > > diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
> > > > index fed4a2d..e9bd6fb 100644
> > > > --- a/include/linux/srcutiny.h
> > > > +++ b/include/linux/srcutiny.h
> > > > @@ -16,6 +16,7 @@
> > > >    struct srcu_struct {
> > > >    	short srcu_lock_nesting[2];	/* srcu_read_lock() nesting depth. */
> > > >    	unsigned short srcu_idx;	/* Current reader array element in bit 0x2. */
> > > > +	unsigned short srcu_idx_max;	/* Furthest future srcu_idx request. */
> > > >    	u8 srcu_gp_running;		/* GP workqueue running? */
> > > >    	u8 srcu_gp_waiting;		/* GP waiting for readers? */
> > > >    	struct swait_queue_head srcu_wq;
> > > > diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> > > > index 3bac1db..b405811 100644
> > > > --- a/kernel/rcu/srcutiny.c
> > > > +++ b/kernel/rcu/srcutiny.c
> > > > @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
> > > >    	ssp->srcu_gp_running = false;
> > > >    	ssp->srcu_gp_waiting = false;
> > > >    	ssp->srcu_idx = 0;
> > > > +	ssp->srcu_idx_max = 0;
> > > >    	INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
> > > >    	INIT_LIST_HEAD(&ssp->srcu_work.entry);
> > > >    	return 0;
> > > > @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp)
> > > >    	struct srcu_struct *ssp;
> > > >    	ssp = container_of(wp, struct srcu_struct, srcu_work);
> > > > -	if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
> > > > +	if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > > >    		return; /* Already running or nothing to do. */
> > > >    	/* Remove recently arrived callbacks and wait for readers. */
> > > > @@ -147,14 +148,19 @@ void srcu_drive_gp(struct work_struct *wp)
> > > >    	 * straighten that out.
> > > >    	 */
> > > >    	WRITE_ONCE(ssp->srcu_gp_running, false);
> > > > -	if (READ_ONCE(ssp->srcu_cb_head))
> > > > +	if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > > 
> > > Should this be USHORT_CMP_LT ?
> > 
> > I believe that you are correct.  As is, it works but does needless
> > grace periods.
> > 
> > > >    		schedule_work(&ssp->srcu_work);
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(srcu_drive_gp);
> > > >    static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > > >    {
> > > > +	unsigned short cookie;
> > > > +
> > > >    	if (!READ_ONCE(ssp->srcu_gp_running)) {
> > > > +		cookie = get_state_synchronize_srcu(ssp);
> > > > +		if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > > > +			WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > > 
> > > I was thinking of a case which might break with this.
> > > 
> > > Consider a scenario, where GP is in progress and kworker is right
> > > before below point, after executing callbacks:
> > > 
> > > void srcu_drive_gp(struct work_struct *wp) {
> > >    <snip>
> > 
> > We updated ->srcu_idx up here, correct?  So it has bottom bit zero.
> > 
> > >    while (lh) {
> > >    <cb execution loop>
> > >    }
> > >    >>> CURRENT EXECUTION POINT
> > 
> > Keeping in mind that Tiny SRCU always runs !PREEMPT, this must be
> > due to an interrupt.
> > 
> Looking more, issue can happen, even when kworker is waiting for GP
> completion @
> 
> swait_event_exclusive(ssp->srcu_wq,
> !READ_ONCE(ssp->srcu_lock_nesting[idx]));
> 
> Other process can call call_srcu() and skip srcu_idx_max update, as
> ssp->srcu_gp_running is true.

Good point!  Does this mean that additional changes are required,
or does the fix below cover this situation as well?

							Thanx, Paul

> Thanks
> Neeraj
> 
> > >    WRITE_ONCE(ssp->srcu_gp_running, false);
> > > 
> > >    if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > >      schedule_work(&ssp->srcu_work);
> > > }
> > > 
> > > Now, at this instance, srcu_gp_start_if_needed() runs and samples
> > > srcu_gp_running and returns, without updating srcu_idx_max
> > > 
> > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > > {
> > >    if (!READ_ONCE(ssp->srcu_gp_running)) returns true
> > >    <snip>
> > > }
> > 
> > This could happen in an interrupt handler, so with you thus far.
> > 
> > > kworker running srcu_drive_gp() resumes and returns without queueing a new
> > > schedule_work(&ssp->srcu_work); for new GP?
> > > 
> > > Prior to this patch, call_srcu() enqueues a cb before entering
> > > srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
> > > queuing, and schedule a work for the new GP, for this scenario.
> > > 
> > > 
> > >   	WRITE_ONCE(ssp->srcu_gp_running, false);
> > > -	if (READ_ONCE(ssp->srcu_cb_head))
> > > +	if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> > >   		schedule_work(&ssp->srcu_work);
> > > 
> > > So, should the "cookie" calculation and "srcu_idx_max" setting be moved
> > > outside of ssp->srcu_gp_running check and maybe return the same cookie to
> > > caller and use that as the returned cookie from
> > > start_poll_synchronize_srcu() ?
> > > 
> > > srcu_gp_start_if_needed()
> > >    cookie = get_state_synchronize_srcu(ssp);
> > >    if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > >       WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > >    if (!READ_ONCE(ssp->srcu_gp_running)) {
> > >    <snip>
> > > }
> > 
> > I believe that you are quite correct, thank you!
> > 
> > But rcutorture does have a call_srcu() (really a ->call, but same if SRCU)
> > in a timer handler.  The race window is quite narrow, so testing it might
> > be a challenge...
> > 
> > This is what I end up with:
> > 
> > 	static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > 	{
> > 		unsigned short cookie;
> > 
> > 		cookie = get_state_synchronize_srcu(ssp);
> > 		if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > 			WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > 		if (!READ_ONCE(ssp->srcu_gp_running)) {
> > 			if (likely(srcu_init_done))
> > 				schedule_work(&ssp->srcu_work);
> > 			else if (list_empty(&ssp->srcu_work.entry))
> > 				list_add(&ssp->srcu_work.entry, &srcu_boot_list);
> > 		}
> > 	}
> > 
> > Does that look plausible?
> 
> Looks good.
> 
> > 
> > 							Thanx, Paul
> > 
> 
> -- 
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