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Subject: Re: [PATCH RFC tip/core/rcu 5/5] srcu: Provide polling interfaces for
 Tree SRCU grace periods
To:     paulmck@kernel.org, rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
        jiangshanlai@gmail.com, akpm@linux-foundation.org,
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References: <20201117004017.GA7444@paulmck-ThinkPad-P72>
 <20201117004052.14758-5-paulmck@kernel.org>
From:   Neeraj Upadhyay <neeraju@codeaurora.org>
Message-ID: <dca110ff-81d1-17b6-06d9-0fdd61363465@codeaurora.org>
Date:   Fri, 20 Nov 2020 17:31:43 +0530
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On 11/17/2020 6:10 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. ]
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>   kernel/rcu/srcutree.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 60 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index d930ece..015d80e 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;
> @@ -822,7 +823,8 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
>   	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 +843,7 @@ 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;
>   }
>   
>   /*
> @@ -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 +1014,60 @@ 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;
> +	smp_mb(); // ^^^

Minor: Should this comment be more descriptive?


Thanks
Neeraj

> +	return true;
> +}
> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
> +
>   /*
>    * Callback function for srcu_barrier() use.
>    */
> 

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