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Date: Mon, 21 Sep 2020 08:34:11 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Marco Elver <elver@...gle.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
neeraj.iitr10@...il.com, rcu@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>,
"Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [RFC v5 0/5] Add support length of each segment in the segcblist
(breaks TREE04)
On Sun, Sep 20, 2020 at 10:06:25PM -0400, Joel Fernandes wrote:
> On Sun, Sep 20, 2020 at 09:21:47PM -0400, Joel Fernandes (Google) wrote:
> >
> > NOTE: I marked as RFC since TREE 04 fails even though TREE03 passes. I don't
> > see any RCU errors in the counters, however when shutdown thread tries to
> > shutdown the system, it hangs when trying to shutdown the rcu_barrier thread.
>
> Looks like if I restore the logic of setting the segcb_list ->len field from
> the callers of extract_done_cbs(), then TREE04 issues go away. I am inclined
> to do that considering this series, for what it is trying to do, does not
> really need to optimize that logic (can be done later).
>
> With the below diff on top of this series, TREE04 passes. I will await any
> other review comments before posting another series.. meanwhile, will pass
> it through the grinders..
As discussed on IRC, I will await v6.
Thanx, Paul
> ---8<---
> diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
> index 319a565f6ecb..d462ae5e340a 100644
> --- a/include/linux/rcu_segcblist.h
> +++ b/include/linux/rcu_segcblist.h
> @@ -76,7 +76,6 @@ struct rcu_segcblist {
> #endif
> u8 enabled;
> u8 offloaded;
> - u8 invoking;
> };
>
> #define RCU_SEGCBLIST_INITIALIZER(n) \
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index ec9a609a461b..3c82c016feb1 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -388,7 +388,6 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> if (rsclp->tails[i] == rsclp->tails[RCU_DONE_TAIL])
> WRITE_ONCE(rsclp->tails[i], &rsclp->head);
> rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
> - rcu_segcblist_add_len(rsclp, -(rclp->len));
> }
>
> /*
> @@ -452,7 +451,17 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
> WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
> rcu_segcblist_set_seglen(rsclp, i, 0);
> }
> - rcu_segcblist_add_len(rsclp, -(rclp->len));
> +}
> +
> +/*
> + * Insert counts from the specified rcu_cblist structure in the
> + * specified rcu_segcblist structure.
> + */
> +void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
> + struct rcu_cblist *rclp)
> +{
> + rcu_segcblist_add_len(rsclp, rclp->len);
> + rclp->len = 0;
> }
>
> /*
> @@ -476,7 +485,6 @@ void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
> break;
> rclp->head = NULL;
> rclp->tail = &rclp->head;
> - rcu_segcblist_add_len(rsclp, rclp->len);
> }
>
> /*
> @@ -492,7 +500,6 @@ void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
> rcu_segcblist_add_seglen(rsclp, RCU_NEXT_TAIL, rclp->len);
> WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rclp->head);
> WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], rclp->tail);
> - rcu_segcblist_add_len(rsclp, rclp->len);
> }
>
> /*
> @@ -637,9 +644,12 @@ void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
> rcu_cblist_init(&donecbs);
> rcu_cblist_init(&pendcbs);
>
> + rcu_segcblist_set_len(src_rsclp, 0);
> rcu_segcblist_extract_done_cbs(src_rsclp, &donecbs);
> rcu_segcblist_extract_pend_cbs(src_rsclp, &pendcbs);
>
> + rcu_segcblist_insert_count(dst_rsclp, &donecbs);
> + rcu_segcblist_insert_count(dst_rsclp, &pendcbs);
> rcu_segcblist_insert_done_cbs(dst_rsclp, &donecbs);
> rcu_segcblist_insert_pend_cbs(dst_rsclp, &pendcbs);
>
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index c2dc08a9b020..15c10d30f88c 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -40,33 +40,14 @@ static inline bool rcu_segcblist_empty(struct rcu_segcblist *rsclp)
> return !READ_ONCE(rsclp->head);
> }
>
> -static inline void rcu_segcblist_set_invoking(struct rcu_segcblist *rsclp)
> -{
> - WRITE_ONCE(rsclp->invoking, 1);
> -}
> -
> -static inline void rcu_segcblist_reset_invoking(struct rcu_segcblist *rsclp)
> -{
> - WRITE_ONCE(rsclp->invoking, 0);
> -}
> -
> /* Return number of callbacks in segmented callback list. */
> static inline long rcu_segcblist_n_cbs(struct rcu_segcblist *rsclp)
> {
> - long ret;
> #ifdef CONFIG_RCU_NOCB_CPU
> - ret = atomic_long_read(&rsclp->len);
> + return atomic_long_read(&rsclp->len);
> #else
> - ret = READ_ONCE(rsclp->len);
> + return READ_ONCE(rsclp->len);
> #endif
> -
> - /*
> - * An invoking list should not appear empty. This is required
> - * by rcu_barrier().
> - */
> - if (ret)
> - return ret;
> - return (READ_ONCE(rsclp->invoking) ? 1 : 0);
> }
>
> /*
> @@ -112,6 +93,8 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> struct rcu_cblist *rclp);
> void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
> struct rcu_cblist *rclp);
> +void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
> + struct rcu_cblist *rclp);
> void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
> struct rcu_cblist *rclp);
> void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> index 62b83fb997ee..0f23d20d485a 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -1196,6 +1196,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
> * schedule another round of callback invocation.
> */
> spin_lock_irq_rcu_node(sdp);
> + rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
> (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
> rcu_seq_snap(&ssp->srcu_gp_seq));
> sdp->srcu_cblist_invoking = false;
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b4dc18e4661c..64dff7b17e74 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2465,7 +2465,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
> }
> trace_rcu_batch_start(rcu_state.name,
> rcu_segcblist_n_cbs(&rdp->cblist), bl);
> - rcu_segcblist_set_invoking(&rdp->cblist);
> rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
> if (offloaded)
> rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
> @@ -2524,9 +2523,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
>
> /* Update counts and requeue any remaining callbacks. */
> rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
> -
> - smp_mb();
> - rcu_segcblist_reset_invoking(&rdp->cblist);
> + smp_mb(); /* List handling before counting for rcu_barrier(). */
> + rcu_segcblist_add_len(&rdp->cblist, -count);
>
> /* Reinstate batch limit if we have worked down the excess. */
> count = rcu_segcblist_n_cbs(&rdp->cblist);
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