call_srcu() has the same constraint as call_rcu(): its callback list and locks are only touched with interrupts disabled. srcu_gp_start_if_needed() enqueues under raw_spin_lock_irqsave() and may walk the srcu_node tree, and callback invocation and grace-period work do likewise. So a call_srcu() with interrupts disabled may be racing a list operation in flight on this CPU, and enqueuing directly could corrupt the list or deadlock.
Defer as call_rcu() does: stage the callback on the srcu_data's ->defer_cbs, chain that srcu_data onto a per-CPU list, and kick a per-CPU irq_work that re-issues it straight to the enqueue helper -- never back through __call_srcu(), so it cannot defer again. The irq_work is global (per CPU, not per srcu_struct) and statically initialized, so the deferral never runs check_init_srcu_struct(). It is IRQ_WORK_INIT_HARD to re-issue promptly, as for call_rcu(). srcu_barrier() and cleanup_srcu_struct() flush it first, and rcutree_migrate_callbacks() calls srcu_offline_drain() to drain an outgoing CPU's deferred callbacks. Gated by CONFIG_RCU_DEFER, as for call_rcu(). Suggested-by: Paul E. McKenney <[email protected]> Signed-off-by: Puranjay Mohan <[email protected]> --- include/linux/srcutree.h | 4 ++ kernel/rcu/rcu.h | 3 + kernel/rcu/srcutree.c | 138 ++++++++++++++++++++++++++++++++++++++- kernel/rcu/tree.c | 2 + 4 files changed, 144 insertions(+), 3 deletions(-) diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h index 75e54e4f963fa..1ce759fb70948 100644 --- a/include/linux/srcutree.h +++ b/include/linux/srcutree.h @@ -13,6 +13,8 @@ #include <linux/rcu_node_tree.h> #include <linux/completion.h> +#include <linux/irq_work_types.h> +#include <linux/llist.h> struct srcu_node; struct srcu_struct; @@ -41,6 +43,8 @@ struct srcu_data { bool srcu_cblist_invoking; /* Invoking these CBs? */ struct timer_list delay_work; /* Delay for CB invoking */ struct work_struct work; /* Context for CB invoking. */ + struct llist_head defer_cbs; /* Callbacks deferred on re-entry. */ + struct llist_node defer_link; /* Links onto the per-CPU deferral drain list */ struct rcu_head srcu_barrier_head; /* For srcu_barrier() use. */ struct rcu_head srcu_ec_head; /* For srcu_expedite_current() use. */ int srcu_ec_state; /* State for srcu_expedite_current(). */ diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index ed6604445f2ac..036d28516e92d 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -587,6 +587,9 @@ static inline bool should_rcu_defer(void) return irqs_disabled() && rcu_scheduler_active != RCU_SCHEDULER_INACTIVE; } +/* Drain an outgoing CPU's deferred SRCU callbacks; see rcutree_migrate_callbacks(). */ +void srcu_offline_drain(int cpu); + enum rcutorture_type { RCU_FLAVOR, RCU_TASKS_FLAVOR, diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 304112674e8a2..b9fe57ff91000 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -20,6 +20,7 @@ #include <linux/percpu.h> #include <linux/preempt.h> #include <linux/irq_work.h> +#include <linux/llist.h> #include <linux/rcupdate_wait.h> #include <linux/sched.h> #include <linux/smp.h> @@ -79,6 +80,46 @@ static void process_srcu(struct work_struct *work); static void srcu_irq_work(struct irq_work *work); static void srcu_delay_timer(struct timer_list *t); +static void srcu_defer_drain(struct irq_work *iw); + +/* + * Per-CPU call_srcu() deferral state. The irq_work is global (shared by every + * srcu_struct), so it must find which srcu_structs deferred callbacks: a + * callback is staged on its srcu_data's ->defer_cbs, and that srcu_data is + * chained (->defer_link) onto this per-CPU ->list that the irq_work walks. + */ +struct srcu_defer { + struct llist_head list; + struct irq_work iw; + raw_spinlock_t lock; +}; + +static DEFINE_PER_CPU(struct srcu_defer, srcu_defer) = { + .lock = __RAW_SPIN_LOCK_UNLOCKED(srcu_defer.lock), + .iw = IRQ_WORK_INIT_HARD(srcu_defer_drain), +}; + +/* + * Register pending deferred call_srcu() callbacks so a following srcu_barrier() + * waits for them. An online CPU's own irq_work re-issues its callbacks, so wait + * it out; an offline CPU's irq_work may never run again, so drain its list + * directly onto this CPU instead. + */ +static void srcu_defer_flush(void) +{ + int cpu; + + if (!IS_ENABLED(CONFIG_RCU_DEFER)) + return; + + for_each_possible_cpu(cpu) { + if (cpu_online(cpu)) + irq_work_sync(&per_cpu(srcu_defer, cpu).iw); + else + srcu_defer_drain(&per_cpu(srcu_defer, cpu).iw); + } +} + /* * Initialize SRCU per-CPU data. Note that statically allocated * srcu_struct structures might already have srcu_read_lock() and @@ -107,6 +148,11 @@ static void init_srcu_struct_data(struct srcu_struct *ssp) sdp->cpu = cpu; INIT_WORK(&sdp->work, srcu_invoke_callbacks); timer_setup(&sdp->delay_work, srcu_delay_timer, 0); + /* + * ->defer_cbs and ->defer_link are valid when zeroed and are not + * reinitialized here, lest we clobber callbacks a reentrant + * call_srcu() already staged. See __call_srcu(). + */ sdp->ssp = ssp; } } @@ -697,6 +743,8 @@ void cleanup_srcu_struct(struct srcu_struct *ssp) return; /* Just leak it! */ /* Wait for irq_work to finish first as it may queue a new work. */ irq_work_sync(&sup->irq_work); + /* Drain any deferred callbacks (shared irq_work) before freeing ->sda. */ + srcu_defer_flush(); flush_delayed_work(&sup->work); for_each_possible_cpu(cpu) { struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu); @@ -1410,8 +1458,18 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, * srcu_read_lock(), and srcu_read_unlock() that are all passed the same * srcu_struct structure. */ -static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp, - rcu_callback_t func, bool do_norm) +/* + * The srcu_cblist and the srcu_node tree are only ever accessed with interrupts + * disabled (srcu_gp_start_if_needed() enqueues under raw_spin_lock_irqsave() and + * may walk the tree; callback invocation and grace-period work likewise). So + * like call_rcu(), enqueuing while a list operation is in flight on this CPU + * would corrupt it or deadlock -- e.g. call_rcu_tasks_trace(), which is + * call_srcu() on a dedicated srcu_struct, from a BPF program. __call_srcu() + * defers whenever interrupts are disabled, via a per-CPU irq_work; only kernels + * where that can happen (CONFIG_RCU_DEFER) need this. + */ +static void srcu_do_enqueue(struct srcu_struct *ssp, struct rcu_head *rhp, + rcu_callback_t func, bool do_norm) { if (debug_rcu_head_queue(rhp)) { /* Probable double call_srcu(), so leak the callback. */ @@ -1423,6 +1481,72 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp, (void)srcu_gp_start_if_needed(ssp, rhp, do_norm); } +static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp, + rcu_callback_t func, bool do_norm) +{ + if (should_rcu_defer()) { + struct srcu_data *sdp = this_cpu_ptr(ssp->sda); + + rhp->func = func; + if (llist_add((struct llist_node *)rhp, &sdp->defer_cbs)) { + /* First deferral on this srcu_data: chain it for the drain. */ + struct srcu_defer *sndp = this_cpu_ptr(&srcu_defer); + + sdp->ssp = ssp; + if (llist_add(&sdp->defer_link, &sndp->list)) + irq_work_queue(&sndp->iw); + } + return; + } + + /* An NMI reaching here entered with irqs enabled, so the enqueue can race. */ + WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi()); + + srcu_do_enqueue(ssp, rhp, func, do_norm); +} + +/* + * Re-issue this CPU's deferred callbacks, going straight to srcu_do_enqueue() so + * they cannot defer again. The irq_work walks its ->list of srcu_data, each + * carrying its own ->defer_cbs. ->lock is held across the drain so drainers + * (irq_work, srcu_defer_flush() and srcu_offline_drain()) serialize and never + * leave a callback off the lists yet not on a callback list. + */ +static void srcu_defer_drain(struct irq_work *iw) +{ + struct srcu_defer *sndp = container_of(iw, struct srcu_defer, iw); + struct llist_node *snode, *snext; + unsigned long flags; + + raw_spin_lock_irqsave(&sndp->lock, flags); + llist_for_each_safe(snode, snext, llist_del_all(&sndp->list)) { + struct srcu_data *sdp = container_of(snode, struct srcu_data, defer_link); + struct srcu_struct *ssp = sdp->ssp; + struct llist_node *cnode, *cnext; + + cnode = llist_del_all(&sdp->defer_cbs); + llist_for_each_safe(cnode, cnext, cnode) { + struct rcu_head *rhp = (struct rcu_head *)cnode; + + srcu_do_enqueue(ssp, rhp, rhp->func, true); + } + } + raw_spin_unlock_irqrestore(&sndp->lock, flags); +} + +/* + * Drain @cpu's deferred call_srcu() callbacks; called from + * rcutree_migrate_callbacks() once @cpu is dead. One pass covers every + * srcu_struct, as the per-CPU srcu_defer list chains them all, and the + * re-issue lands on the current CPU's srcu_data. + */ +void srcu_offline_drain(int cpu) +{ + if (!IS_ENABLED(CONFIG_RCU_DEFER)) + return; + srcu_defer_drain(&per_cpu(srcu_defer, cpu).iw); +} + /** * call_srcu() - Queue a callback for invocation after an SRCU grace period * @ssp: srcu_struct in queue the callback @@ -1677,9 +1801,17 @@ void srcu_barrier(struct srcu_struct *ssp) { int cpu; int idx; - unsigned long s = rcu_seq_snap(&ssp->srcu_sup->srcu_barrier_seq); + unsigned long s; check_init_srcu_struct(ssp); + + /* + * Register any deferred callbacks before snapshotting the sequence. The + * shared irq_work may also drain other srcu_structs', which is harmless. + */ + srcu_defer_flush(); + + s = rcu_seq_snap(&ssp->srcu_sup->srcu_barrier_seq); mutex_lock(&ssp->srcu_sup->srcu_barrier_mutex); if (rcu_seq_done(&ssp->srcu_sup->srcu_barrier_seq, s)) { smp_mb(); /* Force ordering following return. */ diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 93c4242345e67..f8c6930041746 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4627,6 +4627,8 @@ void rcutree_migrate_callbacks(int cpu) * returns; the re-issue lands on this CPU. */ rcu_defer_drain(&rdp->defer_work); + /* Likewise for the outgoing CPU's deferred call_srcu() callbacks. */ + srcu_offline_drain(cpu); if (rcu_rdp_is_offloaded(rdp)) return; -- 2.53.0-Meta
