Re: [PATCH v2 tip/core/rcu 5/6] srcu: Provide polling interfaces for Tree SRCU grace periods

[Neeraj Upadhyay <neeraju@codeaurora.org>]

On 11/21/2020 6:29 AM, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.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@gmail.com>
> [ 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@kernel.org>
> ---

For version in -rcu dev

Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>


Thanks
Neeraj

>   kernel/rcu/srcutiny.c |  2 +-
>   kernel/rcu/srcutree.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++---
>   2 files changed, 64 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
> index b073175..c8f4202 100644
> --- a/kernel/rcu/srcutiny.c
> +++ b/kernel/rcu/srcutiny.c
> @@ -148,7 +148,7 @@ void srcu_drive_gp(struct work_struct *wp)
>   	 * straighten that out.
>   	 */
>   	WRITE_ONCE(ssp->srcu_gp_running, false);
> -	if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
> +	if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
>   		schedule_work(&ssp->srcu_work);
>   }
>   EXPORT_SYMBOL_GPL(srcu_drive_gp);
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index d930ece..945c047 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -810,7 +810,8 @@ static void srcu_leak_callback(struct rcu_head *rhp)
>   /*
>    * Start an SRCU grace period, and also queue the callback if non-NULL.
>    */
> -static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rhp, bool do_norm)
> +static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
> +					     struct rcu_head *rhp, bool do_norm)
>   {
>   	unsigned long flags;
>   	int idx;
> @@ -819,10 +820,12 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
>   	unsigned long s;
>   	struct srcu_data *sdp;
>   
> +	check_init_srcu_struct(ssp);
>   	idx = srcu_read_lock(ssp);
>   	sdp = raw_cpu_ptr(ssp->sda);
>   	spin_lock_irqsave_rcu_node(sdp, flags);
> -	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
> +	if (rhp)
> +		rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
>   	rcu_segcblist_advance(&sdp->srcu_cblist,
>   			      rcu_seq_current(&ssp->srcu_gp_seq));
>   	s = rcu_seq_snap(&ssp->srcu_gp_seq);
> @@ -841,6 +844,7 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
>   	else if (needexp)
>   		srcu_funnel_exp_start(ssp, sdp->mynode, s);
>   	srcu_read_unlock(ssp, idx);
> +	return s;
>   }
>   
>   /*
> @@ -874,7 +878,6 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
>   static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
>   			rcu_callback_t func, bool do_norm)
>   {
> -	check_init_srcu_struct(ssp);
>   	if (debug_rcu_head_queue(rhp)) {
>   		/* Probable double call_srcu(), so leak the callback. */
>   		WRITE_ONCE(rhp->func, srcu_leak_callback);
> @@ -882,7 +885,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
>   		return;
>   	}
>   	rhp->func = func;
> -	srcu_gp_start_if_needed(ssp, rhp, do_norm);
> +	(void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
>   }
>   
>   /**
> @@ -1011,6 +1014,62 @@ void synchronize_srcu(struct srcu_struct *ssp)
>   }
>   EXPORT_SYMBOL_GPL(synchronize_srcu);
>   
> +/**
> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
> + * @ssp: srcu_struct to provide cookie for.
> + *
> + * This function returns a cookie that can be passed to
> + * poll_state_synchronize_srcu(), which will return true if a full grace
> + * period has elapsed in the meantime.  It is the caller's responsibility
> + * to make sure that grace period happens, for example, by invoking
> + * call_srcu() after return from get_state_synchronize_srcu().
> + */
> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
> +{
> +	// Any prior manipulation of SRCU-protected data must happen
> +	// before the load from ->srcu_gp_seq.
> +	smp_mb();
> +	return rcu_seq_snap(&ssp->srcu_gp_seq);
> +}
> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
> +
> +/**
> + * start_poll_synchronize_srcu - Provide cookie and start grace period
> + * @ssp: srcu_struct to provide cookie for.
> + *
> + * This function returns a cookie that can be passed to
> + * poll_state_synchronize_srcu(), which will return true if a full grace
> + * period has elapsed in the meantime.  Unlike get_state_synchronize_srcu(),
> + * this function also ensures that any needed SRCU grace period will be
> + * started.  This convenience does come at a cost in terms of CPU overhead.
> + */
> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
> +{
> +	return srcu_gp_start_if_needed(ssp, NULL, true);
> +}
> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
> +
> +/**
> + * poll_state_synchronize_srcu - Has cookie's grace period ended?
> + * @ssp: srcu_struct to provide cookie for.
> + * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
> + *
> + * This function takes the cookie that was returned from either
> + * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
> + * returns @true if an SRCU grace period elapsed since the time that the
> + * cookie was created.
> + */
> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
> +{
> +	if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
> +		return false;
> +	// Ensure that the end of the SRCU grace period happens before
> +	// any subsequent code that the caller might execute.
> +	smp_mb(); // ^^^
> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> +
>   /*
>    * Callback function for srcu_barrier() use.
>    */
> 

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