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SubjectRe: [PATCH RFC tip/core/rcu 3/5] srcu: Provide internal interface to start a Tree SRCU grace period
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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.
> This polling needs to initiate an SRCU grace period without having
> to queue (and manage) a callback. This commit therefore splits the
> Tree SRCU __call_srcu() function into callback-initialization and
> queuing/start-grace-period portions, with the latter in a new function
> named srcu_gp_start_if_needed(). This function may be passed a NULL
> callback pointer, in which case it will refrain from queuing anything.
>
> Why have the new function mess with queuing? Locking considerations,
> of course!
>
> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---

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


Thanks
Neeraj

> kernel/rcu/srcutree.c | 66 +++++++++++++++++++++++++++++----------------------
> 1 file changed, 37 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 79b7081..d930ece 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -808,6 +808,42 @@ 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)
> +{
> + unsigned long flags;
> + int idx;
> + bool needexp = false;
> + bool needgp = false;
> + unsigned long s;
> + struct srcu_data *sdp;
> +
> + 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);
> + rcu_segcblist_advance(&sdp->srcu_cblist,
> + rcu_seq_current(&ssp->srcu_gp_seq));
> + s = rcu_seq_snap(&ssp->srcu_gp_seq);
> + (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
> + if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> + sdp->srcu_gp_seq_needed = s;
> + needgp = true;
> + }
> + if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
> + sdp->srcu_gp_seq_needed_exp = s;
> + needexp = true;
> + }
> + spin_unlock_irqrestore_rcu_node(sdp, flags);
> + if (needgp)
> + srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> + else if (needexp)
> + srcu_funnel_exp_start(ssp, sdp->mynode, s);
> + srcu_read_unlock(ssp, idx);
> +}
> +
> +/*
> * Enqueue an SRCU callback on the srcu_data structure associated with
> * the current CPU and the specified srcu_struct structure, initiating
> * grace-period processing if it is not already running.
> @@ -838,13 +874,6 @@ static void srcu_leak_callback(struct rcu_head *rhp)
> static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> rcu_callback_t func, bool do_norm)
> {
> - unsigned long flags;
> - int idx;
> - bool needexp = false;
> - bool needgp = false;
> - unsigned long s;
> - struct srcu_data *sdp;
> -
> check_init_srcu_struct(ssp);
> if (debug_rcu_head_queue(rhp)) {
> /* Probable double call_srcu(), so leak the callback. */
> @@ -853,28 +882,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
> return;
> }
> rhp->func = func;
> - 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);
> - rcu_segcblist_advance(&sdp->srcu_cblist,
> - rcu_seq_current(&ssp->srcu_gp_seq));
> - s = rcu_seq_snap(&ssp->srcu_gp_seq);
> - (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
> - if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
> - sdp->srcu_gp_seq_needed = s;
> - needgp = true;
> - }
> - if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
> - sdp->srcu_gp_seq_needed_exp = s;
> - needexp = true;
> - }
> - spin_unlock_irqrestore_rcu_node(sdp, flags);
> - if (needgp)
> - srcu_funnel_gp_start(ssp, sdp, s, do_norm);
> - else if (needexp)
> - srcu_funnel_exp_start(ssp, sdp->mynode, s);
> - srcu_read_unlock(ssp, idx);
> + srcu_gp_start_if_needed(ssp, rhp, do_norm);
> }
>
> /**
>

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