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Date: Thu, 5 Aug 2021 15:12:11 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Andrew Morton <akpm@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Stefan Metzmacher <metze@...ba.org>, stable@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] Fix: tracepoint: rcu get state and cond sync for
static call updates (v2)
On Thu, 5 Aug 2021 09:27:17 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> State transitions from 1->0->1 and N->2->1 callbacks require RCU
> synchronization. Rather than performing the RCU synchronization every
> time the state change occurs, which is quite slow when many tracepoints
> are registered in batch, instead keep a snapshot of the RCU state on the
> most recent transitions which belong to a chain, and conditionally wait
> for a grace period on the last transition of the chain if one g.p. has
> not elapsed since the last snapshot.
>
> This applies to both RCU and SRCU.
>
> Link: https://lore.kernel.org/io-uring/4ebea8f0-58c9-e571-fd30-0ce4f6f09c70@samba.org/
> Fixes: d25e37d89dd2 ("tracepoint: Optimize using static_call()")
> Fixes: 547305a64632 ("tracepoint: Fix out of sync data passing by static caller")
> Fixes: 352384d5c84e ("tracepoints: Update static_call before tp_funcs when adding a tracepoint")
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: "Paul E. McKenney" <paulmck@...nel.org>
> Cc: Stefan Metzmacher <metze@...ba.org>
> Cc: <stable@...r.kernel.org> # 5.10+
> ---
> Changes since v1:
> - Use tp_rcu_get_state/tp_rcu_cond_sync on 2->1 transition when
> tp_funcs[0].data != old[0].data rather than
> tracepoint_synchronize_unregister.
> ---
> kernel/tracepoint.c | 81 ++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 69 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 8d772bd6894d..d8f69580001c 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -28,6 +28,44 @@ extern tracepoint_ptr_t __stop___tracepoints_ptrs[];
> DEFINE_SRCU(tracepoint_srcu);
> EXPORT_SYMBOL_GPL(tracepoint_srcu);
>
> +enum tp_transition_sync {
> + TP_TRANSITION_SYNC_1_0_1,
> + TP_TRANSITION_SYNC_N_2_1,
> +
> + _NR_TP_TRANSITION_SYNC,
> +};
> +
> +struct tp_transition_snapshot {
> + unsigned long rcu;
> + unsigned long srcu;
> + bool ongoing;
> +};
> +
> +/* Protected by tracepoints_mutex */
> +static struct tp_transition_snapshot tp_transition_snapshot[_NR_TP_TRANSITION_SYNC];
> +
> +static void tp_rcu_get_state(enum tp_transition_sync sync)
> +{
> + struct tp_transition_snapshot *snapshot = &tp_transition_snapshot[sync];
> +
> + /* Keep the latest get_state snapshot. */
> + snapshot->rcu = get_state_synchronize_rcu();
> + snapshot->srcu = start_poll_synchronize_srcu(&tracepoint_srcu);
> + snapshot->ongoing = true;
> +}
> +
> +static void tp_rcu_cond_sync(enum tp_transition_sync sync)
> +{
> + struct tp_transition_snapshot *snapshot = &tp_transition_snapshot[sync];
> +
> + if (!snapshot->ongoing)
> + return;
> + cond_synchronize_rcu(snapshot->rcu);
> + if (!poll_state_synchronize_srcu(&tracepoint_srcu, snapshot->srcu))
> + synchronize_srcu(&tracepoint_srcu);
> + snapshot->ongoing = false;
> +}
> +
> /* Set to 1 to enable tracepoint debug output */
> static const int tracepoint_debug;
>
> @@ -311,6 +349,11 @@ static int tracepoint_add_func(struct tracepoint *tp,
> */
> switch (nr_func_state(tp_funcs)) {
> case TP_FUNC_1: /* 0->1 */
> + /*
> + * Make sure new static func never uses old data after a
> + * 1->0->1 transition sequence.
> + */
> + tp_rcu_cond_sync(TP_TRANSITION_SYNC_1_0_1);
> /* Set static call to first function */
> tracepoint_update_call(tp, tp_funcs);
> /* Both iterator and static call handle NULL tp->funcs */
> @@ -326,9 +369,21 @@ static int tracepoint_add_func(struct tracepoint *tp,
> * static call update/call.
> */
> rcu_assign_pointer(tp->funcs, tp_funcs);
> + /*
> + * Make sure static func never uses incorrect data after a
> + * 1->...->2->1 transition sequence.
> + */
> + if (tp_funcs[0].data != old[0].data)
> + tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
> break;
> case TP_FUNC_N: /* N->N+1 (N>1) */
> rcu_assign_pointer(tp->funcs, tp_funcs);
> + /*
> + * Make sure static func never uses incorrect data after a
> + * N->...->2->1 (N>1) transition sequence.
> + */
> + if (tp_funcs[0].data != old[0].data)
> + tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
> break;
Looks to me that the above can be replaced with:
case TP_FUNC_2: /* 1->2 */
/* Set iterator static call */
tracepoint_update_call(tp, tp_funcs);
/*
* Iterator callback installed before updating tp->funcs.
* Requires ordering between RCU assign/dereference and
* static call update/call.
*/
fallthrough;
case TP_FUNC_N: /* N->N+1 (N>1) */
rcu_assign_pointer(tp->funcs, tp_funcs);
/*
* Make sure static func never uses incorrect data after a
* N->...->2->1 (N>1) transition sequence.
*/
if (tp_funcs[0].data != old[0].data)
tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
break;
> default:
> WARN_ON_ONCE(1);
> @@ -372,24 +427,20 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> /* Both iterator and static call handle NULL tp->funcs */
> rcu_assign_pointer(tp->funcs, NULL);
> /*
> - * Make sure new func never uses old data after a 1->0->1
> - * transition sequence.
> - * Considering that transition 0->1 is the common case
> - * and don't have rcu-sync, issue rcu-sync after
> - * transition 1->0 to break that sequence by waiting for
> - * readers to be quiescent.
> + * Make sure new static func never uses old data after a
> + * 1->0->1 transition sequence.
> */
> - tracepoint_synchronize_unregister();
> + tp_rcu_get_state(TP_TRANSITION_SYNC_1_0_1);
> break;
> case TP_FUNC_1: /* 2->1 */
> rcu_assign_pointer(tp->funcs, tp_funcs);
> /*
> - * On 2->1 transition, RCU sync is needed before setting
> - * static call to first callback, because the observer
> - * may have loaded any prior tp->funcs after the last one
> - * associated with an rcu-sync.
> + * Make sure static func never uses incorrect data after a
> + * N->...->2->1 (N>2) transition sequence.
> */
> - tracepoint_synchronize_unregister();
We should add a comment here with:
/*
* If the first element's data has changed, then force the
* synchronization, to prevent current readers that have loaded
* the old data from calling the new function.
*/
Can you send a v3 of just this patch? I'll pull in the other patches.
-- Steve
> + if (tp_funcs[0].data != old[0].data)
> + tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
> + tp_rcu_cond_sync(TP_TRANSITION_SYNC_N_2_1);
> /* Set static call to first function */
> tracepoint_update_call(tp, tp_funcs);
> break;
> @@ -397,6 +448,12 @@ static int tracepoint_remove_func(struct tracepoint *tp,
> fallthrough;
> case TP_FUNC_N:
> rcu_assign_pointer(tp->funcs, tp_funcs);
> + /*
> + * Make sure static func never uses incorrect data after a
> + * N->...->2->1 (N>2) transition sequence.
> + */
> + if (tp_funcs[0].data != old[0].data)
> + tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
> break;
> default:
> WARN_ON_ONCE(1);
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