Give Tiny call_rcu() the same treatment as Tree RCU. When interrupts are disabled and the scheduler is up, stage the callback on a lockless list that an irq_work re-issues later. One global list and irq_work suffice since Tiny RCU is uniprocessor, and there is no CPU-offline drain.
The re-issue runs with interrupts disabled and can be re-entered by instrumentation, so a draining flag drops a deferring call_rcu() seen mid-drain (unless from an NMI), as in Tree RCU. Gated by CONFIG_RCU_DEFER, though the deferral state is unconditional. Interrupts stay off for the whole batch, but the re-issue is a tail append with no locks. TINY_RCU implies !SMP, where arch_irq_work_has_interrupt() is false, so the drain always waits for the tick and a batch is whatever one tick's worth of interrupts-disabled call_rcu()s staged. As in Tree RCU the drain clears ->next before re-issuing, which bounds a node self-linked by a double call_rcu(): rcu_do_enqueue()'s duplicate path returns without clearing it. A longer cycle is not bounded; a double call_rcu() stays undefined. The idle-task reschedule moves out of the enqueue helper so that a drain does it once for the batch rather than once per callback, which would otherwise take the runqueue lock N times with interrupts disabled. Suggested-by: Paul E. McKenney <[email protected]> Signed-off-by: Puranjay Mohan <[email protected]> --- kernel/rcu/tiny.c | 127 +++++++++++++++++++++++++++++++++++++--------- 1 file changed, 104 insertions(+), 23 deletions(-) diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c index dccccd6be9411..656b6a682e31a 100644 --- a/kernel/rcu/tiny.c +++ b/kernel/rcu/tiny.c @@ -11,6 +11,8 @@ */ #include <linux/completion.h> #include <linux/interrupt.h> +#include <linux/irq_work.h> +#include <linux/llist.h> #include <linux/notifier.h> #include <linux/rcupdate_wait.h> #include <linux/kernel.h> @@ -42,8 +44,100 @@ static struct rcu_ctrlblk rcu_ctrlblk = { .gp_seq = 0 - 300UL, }; +/* + * The callback list is only accessed with interrupts disabled, so a call_rcu() + * that arrives with interrupts off stages the callback on a lockless list that + * an irq_work re-issues later. One global list and irq_work suffice, as Tiny + * RCU is uniprocessor. + */ +static void rcu_defer_drain(struct irq_work *iw); +static LLIST_HEAD(rcu_defer_list); +static struct irq_work rcu_defer_iw = IRQ_WORK_INIT_HARD(rcu_defer_drain); +static bool rcu_defer_draining; + +/* + * Also called by __rcu_defer_drain() to re-issue a deferred callback, so it + * must not re-check the deferral condition. + */ +static void rcu_do_enqueue(struct rcu_head *head, rcu_callback_t func) +{ + static atomic_t doublefrees; + unsigned long flags; + + if (debug_rcu_head_queue(head)) { + if (atomic_inc_return(&doublefrees) < 4) { + pr_err("%s(): Double-freed CB %p->%pS()!!! ", __func__, head, head->func); + mem_dump_obj(head); + } + return; + } + + head->func = func; + head->next = NULL; + + local_irq_save(flags); + *rcu_ctrlblk.curtail = head; + rcu_ctrlblk.curtail = &head->next; + local_irq_restore(flags); +} + +/* Force scheduling for rcu_qs() when enqueuing from the idle task. */ +static void rcu_resched_if_idle(void) +{ + if (unlikely(is_idle_task(current))) + resched_cpu(0); +} + +static void __rcu_defer_drain(void) +{ + struct llist_node *node, *next; + bool drained = false; + unsigned long flags; + + if (!IS_ENABLED(CONFIG_RCU_DEFER)) + return; + + /* Re-issued newest-first; nothing depends on call_rcu() ordering. */ + local_irq_save(flags); + llist_for_each_safe(node, next, llist_del_all(&rcu_defer_list)) { + struct rcu_head *head = (struct rcu_head *)node; + + /* Bounds a node self-linked by a double call_rcu(). */ + head->next = NULL; + rcu_do_enqueue(head, head->func); + drained = true; + } + local_irq_restore(flags); + + if (drained) + rcu_resched_if_idle(); +} + +/* Only the irq_work drain can be re-fed by its own re-issue; see Tree RCU. */ +static void rcu_defer_drain(struct irq_work *iw) +{ + WRITE_ONCE(rcu_defer_draining, true); + __rcu_defer_drain(); + WRITE_ONCE(rcu_defer_draining, false); +} + +static void call_rcu_defer(struct rcu_head *head, rcu_callback_t func) +{ + /* A re-entrant call_rcu() during the drain would livelock it; drop it. */ + if (READ_ONCE(rcu_defer_draining) && !in_nmi()) { + WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU), + "call_rcu() re-entered during callback drain; leaking callback\n"); + return; + } + head->func = func; + if (llist_add((struct llist_node *)head, &rcu_defer_list)) + irq_work_queue(&rcu_defer_iw); +} + void rcu_barrier(void) { + /* Register any deferred callbacks so the wait below covers them. */ + __rcu_defer_drain(); wait_rcu_gp(call_rcu_hurry); } EXPORT_SYMBOL(rcu_barrier); @@ -157,29 +251,19 @@ EXPORT_SYMBOL_GPL(synchronize_rcu); */ void call_rcu(struct rcu_head *head, rcu_callback_t func) { - static atomic_t doublefrees; - unsigned long flags; - - if (debug_rcu_head_queue(head)) { - if (atomic_inc_return(&doublefrees) < 4) { - pr_err("%s(): Double-freed CB %p->%pS()!!! ", __func__, head, head->func); - mem_dump_obj(head); - } + if (should_rcu_defer()) { + call_rcu_defer(head, func); return; } - head->func = func; - head->next = NULL; + /* + * Only reachable from an NMI when deferral is off: before the scheduler + * is up, or with CONFIG_RCU_DEFER=n. The enqueue can then race. + */ + WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi()); - local_irq_save(flags); - *rcu_ctrlblk.curtail = head; - rcu_ctrlblk.curtail = &head->next; - local_irq_restore(flags); - - if (unlikely(is_idle_task(current))) { - /* force scheduling for rcu_qs() */ - resched_cpu(0); - } + rcu_do_enqueue(head, func); + rcu_resched_if_idle(); } EXPORT_SYMBOL_GPL(call_rcu); @@ -211,10 +295,7 @@ unsigned long start_poll_synchronize_rcu(void) { unsigned long gp_seq = get_state_synchronize_rcu(); - if (unlikely(is_idle_task(current))) { - /* force scheduling for rcu_qs() */ - resched_cpu(0); - } + rcu_resched_if_idle(); return gp_seq; } EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu); -- 2.53.0-Meta
