Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods

[Paul E. McKenney]

On Tue, Nov 24, 2020 at 10:44:24AM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 11/24/2020 2:42 AM, Paul E. McKenney wrote:
> > On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote:
> > > 
> > > 
> > > On 11/21/2020 6:29 AM, paulmck@xxxxxxxxxx wrote:
> > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > > 
> > > > 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@xxxxxxxxxxxxxx/
> > > > Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > > > [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
> > > > [ paulmck: Apply feedback from Neeraj Upadhyay. ]
> > > > Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
> > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > > > ---
> > > >    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 d9edb67..c7f0c1f 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..b073175 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;
> > > 
> > > Minor: cleanup_srcu_struct() can probably have 2 new sanity checks?
> > > 
> > > WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
> > > WARN_ON(ssp->srcu_idx & 1);
> > 
> > Good point, added and under test.
> > 
> > > Thanks
> > > Neeraj
> > > 
> > > > @@ -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. */
> > > > @@ -147,13 +148,18 @@ 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)))
> > > >    		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;
> > > > +
> > > > +	cookie = get_state_synchronize_srcu(ssp);
> > > > +	if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
> > > > +		WRITE_ONCE(ssp->srcu_idx_max, cookie);
> > > 
> > > Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid
> > > scheduling work?
> > 
> > How about like this?
> 
> Looks good!

Are you willing to give an ack or reviewed-by for either patch?

							Thanx, Paul

> Thanks
> Neeraj
> 
> > 
> > 	static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
> > 	{
> > 		unsigned short cookie;
> > 
> > 		cookie = get_state_synchronize_srcu(ssp);
> > 		if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
> > 			return;
> > 		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);
> > 		}
> > 	}
> > 
> > Testing this next.  ;-)
> > 
> > 							Thanx, Paul
> > 
> > > Thanks
> > > Neeraj
> > > 
> > > >    	if (!READ_ONCE(ssp->srcu_gp_running)) {
> > > >    		if (likely(srcu_init_done))
> > > >    			schedule_work(&ssp->srcu_work);
> > > > @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(synchronize_srcu);
> > > > +/*
> > > > + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> > > > + */
> > > > +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> > > > +{
> > > > +	unsigned long ret;
> > > > +
> > > > +	barrier();
> > > > +	ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
> > > > +	barrier();
> > > > +	return ret & USHRT_MAX;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> > > > +
> > > > +/*
> > > > + * start_poll_synchronize_srcu - Provide cookie and start grace period
> > > > + *
> > > > + * The difference between this and get_state_synchronize_srcu() is that
> > > > + * this function ensures that the poll_state_synchronize_srcu() will
> > > > + * eventually return the value true.
> > > > + */
> > > > +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> > > > +{
> > > > +	unsigned long ret = get_state_synchronize_srcu(ssp);
> > > > +
> > > > +	srcu_gp_start_if_needed(ssp);
> > > > +	return ret;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> > > > +
> > > > +/*
> > > > + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> > > > + */
> > > > +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> > > > +{
> > > > +	bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
> > > > +
> > > > +	barrier();
> > > > +	return ret;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> > > > +
> > > >    /* Lockdep diagnostics.  */
> > > >    void __init rcu_scheduler_starting(void)
> > > >    {
> > > > 
> > > 
> > > -- 
> > > QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> > > the Code Aurora Forum, hosted by The Linux Foundation
> 
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation