Date:   Fri, 20 Nov 2020 17:28:32 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     paulmck@...nel.org, rcu@...r.kernel.org
Cc:     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

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 ?

>   		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>

   while (lh) {
   <cb execution loop>
   }
   >>> CURRENT EXECUTION POINT

   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>
}

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>
}


Thanks
Neeraj

>   		if (likely(srcu_init_done))
>   			schedule_work(&ssp->srcu_work);
>   		else if (list_empty(&ssp->srcu_work.entry))
> @@ -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)
>   {
> 

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