Batch concurrent hazptr_synchronize() callers into a shared scan cycle, avoiding redundant scans of the per-CPU slots.
Queue waiters to a kthread and let each scan cycle make one pass over all CPUs. Each waiter tracks per-CPU progress for both wildcard generations, allowing multiple waiters to share the same scan. Flip the wildcard before scanning. New acquires then use the new generation, so the old-generation mask makes forward progress even under a steady stream of readers. Waiters that remain blocked are retried after a short delay. Fall back to the existing direct two-phase scan if the scan kthread is unavailable or waiter state cannot be allocated. Signed-off-by: Kunwu Chan <[email protected]> --- kernel/hazptr.c | 274 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 274 insertions(+) diff --git a/kernel/hazptr.c b/kernel/hazptr.c index d3d1050d92cf..ce553a61b119 100644 --- a/kernel/hazptr.c +++ b/kernel/hazptr.c @@ -12,6 +12,10 @@ #include <linux/mutex.h> #include <linux/list.h> #include <linux/export.h> +#include <linux/completion.h> +#include <linux/kthread.h> +#include <linux/slab.h> +#include <linux/swait.h> /* * The current hazard pointer wildcard. Flips between 1UL and 2UL to guarantee @@ -209,12 +213,251 @@ void hazptr_scan_period(void *addr, void *scan_wildcard) } } +/* + * Batch hazptr_synchronize() callers through a shared scan kthread. + */ + +struct hazptr_waiter { + struct list_head node; + void *addr; + struct completion done; + /* + * Per-wildcard-generation progress masks. A CPU bit is + * cleared when the scan observes neither @addr nor that + * generation's wildcard on the CPU. + */ + unsigned long *cpu_mask; /* 2 * BITS_TO_LONGS(nr_cpu_ids) */ +}; + +/* Return waiter @w's progress mask for wildcard generation @gen. */ +static unsigned long *hazptr_waiter_mask(struct hazptr_waiter *w, int gen) +{ + return w->cpu_mask + gen * BITS_TO_LONGS(nr_cpu_ids); +} + +struct hazptr_scan_state { + struct task_struct *kthread; + struct swait_queue_head wq; + bool wakeup; + struct mutex lock; + struct list_head pending; + struct list_head scanning; /* kthread only */ +}; +static struct hazptr_scan_state hazptr_scan; + +/* + * Check a CPU's overflow lists. A backup slot can hold a wildcard + * because __hazptr_acquire() writes the wildcard to any slot, + * including backup slots from hazptr_chain_backup_slot(). + * + * @addr: address the waiter is waiting on + * @old_wc: wildcard value of the pre-flip generation + * @new_wc: wildcard value of the post-flip generation + * @has_old: set if any overflow slot holds @old_wc + * @has_new: set if any overflow slot holds @new_wc + * + * Returns true if @addr is present. + */ +static bool hazptr_ovf_list_blocked(int cpu, void *addr, + void *old_wc, void *new_wc, + bool *has_old, bool *has_new) +{ + struct hazptr_overflow_list_flip *ovf = per_cpu_ptr(&percpu_overflow_list_flip, cpu); + bool found_addr = false; + int i; + + for (i = 0; i < 2; i++) { + struct hazptr_overflow_list *list = &ovf->array[i]; + struct hazptr_backup_slot *b; + unsigned long flags; + + raw_spin_lock_irqsave(&list->lock, flags); + hlist_for_each_entry(b, &list->head, overflow_node) { + /* Pairs with smp_store_release in hazptr_release(). */ + void *val = smp_load_acquire(&b->slot.addr); + + if (val == addr) + found_addr = true; + else if (val == old_wc) + *has_old = true; + else if (val == new_wc) + *has_new = true; + } + raw_spin_unlock_irqrestore(&list->lock, flags); + } + return found_addr; +} + +/* + * Move pending waiters to ->scanning, flip the wildcard, then make + * one pass over all CPUs. Clear per-waiter bits for CPUs that no + * longer hold the waiter address or the corresponding wildcard. + * + * After the flip, new acquires use the new wildcard. The old + * generation therefore makes forward progress and is fully cleared + * after enough scan cycles. + */ +static void hazptr_scan_do_cycle(void) +{ + void *old_wc, *new_wc; + unsigned int old_idx, new_idx; + int cpu; + struct hazptr_waiter *w, *n; + LIST_HEAD(done); + + mutex_lock(&hazptr_wildcard_lock); + + mutex_lock(&hazptr_scan.lock); + list_splice_tail_init(&hazptr_scan.pending, &hazptr_scan.scanning); + mutex_unlock(&hazptr_scan.lock); + + if (list_empty(&hazptr_scan.scanning)) { + mutex_unlock(&hazptr_wildcard_lock); + return; + } + + old_wc = READ_ONCE(hazptr_wildcard); + new_wc = flip_wildcard(old_wc); + WRITE_ONCE(hazptr_wildcard, new_wc); + old_idx = (unsigned long)old_wc - 1; + new_idx = 1 - old_idx; + + /* + * One pass over all CPUs for the per-CPU slots, checking + * overflow lists for the remaining waiters. + */ + for_each_possible_cpu(cpu) { + struct hazptr_percpu_slots *slots = per_cpu_ptr(&hazptr_percpu_slots, cpu); + void *vals[NR_HAZPTR_PERCPU_SLOTS]; + bool has_old = false, has_new = false; + unsigned int idx; + + for (idx = 0; idx < NR_HAZPTR_PERCPU_SLOTS; idx++) { + /* Pairs with smp_store_release in hazptr_release(). */ + vals[idx] = smp_load_acquire(&slots->items[idx].slot.addr); + if (vals[idx] == old_wc) + has_old = true; + else if (vals[idx] == new_wc) + has_new = true; + } + + list_for_each_entry(w, &hazptr_scan.scanning, node) { + bool has_addr = false; + + if (!test_bit(cpu, hazptr_waiter_mask(w, old_idx)) && + !test_bit(cpu, hazptr_waiter_mask(w, new_idx))) + continue; /* Both bits already clear. */ + for (idx = 0; idx < NR_HAZPTR_PERCPU_SLOTS; idx++) { + if (vals[idx] == w->addr) { + has_addr = true; + break; + } + } + if (!has_addr) + has_addr = hazptr_ovf_list_blocked(cpu, w->addr, + old_wc, new_wc, &has_old, &has_new); + if (has_addr) + continue; + if (!has_old) + __clear_bit(cpu, hazptr_waiter_mask(w, old_idx)); + if (!has_new) + __clear_bit(cpu, hazptr_waiter_mask(w, new_idx)); + } + } + + mutex_unlock(&hazptr_wildcard_lock); + + /* Complete waiters whose masks are both empty. */ + list_for_each_entry_safe(w, n, &hazptr_scan.scanning, node) { + if (bitmap_empty(hazptr_waiter_mask(w, 0), nr_cpu_ids) && + bitmap_empty(hazptr_waiter_mask(w, 1), nr_cpu_ids)) + list_move(&w->node, &done); + } + + list_for_each_entry_safe(w, n, &done, node) { + list_del_init(&w->node); + complete(&w->done); + } +} + +/* + * Shared scan kthread for hazptr_synchronize() waiters. + */ +static int hazptr_scan_kthread(void *unused) +{ + for (;;) { + bool idle; + + swait_event_idle_exclusive(hazptr_scan.wq, + READ_ONCE(hazptr_scan.wakeup)); + + hazptr_scan_do_cycle(); + + mutex_lock(&hazptr_scan.lock); + idle = list_empty(&hazptr_scan.pending) && + list_empty(&hazptr_scan.scanning); + if (idle) + WRITE_ONCE(hazptr_scan.wakeup, false); + mutex_unlock(&hazptr_scan.lock); + + if (idle) + continue; + /* Waiters still blocked: retry after a polling delay. */ + schedule_timeout_idle(1); + } + return 0; +} + +/* + * Queue @addr for scan-thread processing, then sleep until the scan + * thread observes that @addr is no longer held by any hazard pointer. + * Returns false if the waiter masks cannot be allocated, in which + * case the caller falls back to the direct scan. + */ +static bool hazptr_synchronize_queued(void *addr) +{ + struct hazptr_waiter waiter = { + .addr = addr, + }; + unsigned long *masks; + unsigned int mask_longs = BITS_TO_LONGS(nr_cpu_ids); + + masks = kcalloc(2, mask_longs * sizeof(unsigned long), GFP_KERNEL); + if (!masks) + return false; + bitmap_fill(masks, nr_cpu_ids); + bitmap_fill(masks + mask_longs, nr_cpu_ids); + waiter.cpu_mask = masks; + + init_completion(&waiter.done); + INIT_LIST_HEAD(&waiter.node); + + /* Enqueue and wake the scan kthread. */ + mutex_lock(&hazptr_scan.lock); + list_add_tail(&waiter.node, &hazptr_scan.pending); + if (!READ_ONCE(hazptr_scan.wakeup)) { + WRITE_ONCE(hazptr_scan.wakeup, true); + swake_up_one(&hazptr_scan.wq); + } + mutex_unlock(&hazptr_scan.lock); + + /* Sleep until the scan thread completes this waiter. */ + wait_for_completion(&waiter.done); + kfree(masks); + return true; +} + /* * hazptr_synchronize: Wait until @addr is released from all slots. * * Wait to observe that each slot contains a value that differs from * @addr before returning. * Should be called from preemptible context. + * + * If the scan kthread is running, the caller is queued and the scan + * thread performs the work, allowing multiple concurrent callers to + * share a single scan cycle. Otherwise, the existing direct + * two-phase scan is used as a fallback. */ void hazptr_synchronize(void *addr) { @@ -235,6 +478,13 @@ void hazptr_synchronize(void *addr) /* Memory ordering: Store A before Load B. */ smp_mb(); + /* Use the scan thread if available. */ + /* Pairs with smp_store_release in hazptr_scan_init(). */ + if (smp_load_acquire(&hazptr_scan.kthread) && + hazptr_synchronize_queued(addr)) + return; + + /* Fallback: direct two-phase wildcard scan. */ guard(mutex)(&hazptr_wildcard_lock); scan_wildcard = flip_wildcard(hazptr_wildcard); hazptr_scan_period(addr, scan_wildcard); @@ -282,3 +532,27 @@ void __init hazptr_init(void) } } } + +/* + * Initialize the scan kthread. On failure falls back to the direct + * scan (busy-wait) path at synchronize time. + * core_initcall ensures the scheduler is ready before kthread_run. + */ +static int __init hazptr_scan_init(void) +{ + struct task_struct *t; + + init_swait_queue_head(&hazptr_scan.wq); + mutex_init(&hazptr_scan.lock); + INIT_LIST_HEAD(&hazptr_scan.pending); + INIT_LIST_HEAD(&hazptr_scan.scanning); + + t = kthread_run(hazptr_scan_kthread, NULL, "hazptr_scan"); + if (!IS_ERR(t)) + /* Pairs with smp_load_acquire in hazptr_synchronize(). */ + smp_store_release(&hazptr_scan.kthread, t); + else + pr_warn("hazptr: scan thread failed, using direct scan\n"); + return 0; +} +core_initcall(hazptr_scan_init); -- 2.43.0
