On Fri, Sep 18, 2026 at 05:00:54PM -0700, Paul E. McKenney wrote:
> From: Mathieu Desnoyers <[email protected]>
>
> Implement a two-phase wildcard scan to guarantee forward progress of
> synchronize_hazptr() even if there is a steady stream of ill-timed
> readers which populate wildcards into per-CPU slots.
>
> This is performed by flipping between two wildcard values (1UL and 2UL),
> and alternatively scanning for the opposite wildcard while newcoming
> readers use the other one.
>
> There is no possibility to miss a reader because all slots for all
> wildcards are accounted for during a synchronize.
>
> As a simplification, use this period flip to drive the hazptr overflow
> list selection as well, since there is really no point is making the
> overflow list flip use a different state.
>
> Protect the wildcard flip with a mutex.
>
> Signed-off-by: Mathieu Desnoyers <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
> Cc: Boqun Feng <[email protected]>
> Reviewed-by: Bradley Morgan <[email protected]>
> ---
> include/linux/hazptr.h | 6 ++-
> kernel/hazptr.c | 98 ++++++++++++++++++++++++++++++------------
> 2 files changed, 74 insertions(+), 30 deletions(-)
>
> diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h
> index 43998bf43de4..43122c5673bd 100644
> --- a/include/linux/hazptr.h
> +++ b/include/linux/hazptr.h
> @@ -28,7 +28,9 @@
>
> /* 4 slots (each sizeof(hazptr_slot_item)) fit in a single 64-byte cache
> line. */
> #define NR_HAZPTR_PERCPU_SLOTS 4
> -#define HAZPTR_WILDCARD ((void *) 0x1UL)
> +
> +/* The current hazard pointer wildcard. */
> +extern void *hazptr_wildcard;
>
> /*
> * Hazard pointer slot.
> @@ -243,7 +245,7 @@ void *hazptr_acquire(struct hazptr_ctx *ctx, void * const
> *addr_p)
> #endif
> if (unlikely(slot->addr))
> return __hazptr_acquire(ctx, addr_p);
> - WRITE_ONCE(slot->addr, HAZPTR_WILDCARD); /* Store B */
> + WRITE_ONCE(slot->addr, READ_ONCE(hazptr_wildcard)); /* Store B */
>
> /* Memory ordering: Store B before Load A. */
> smp_mb();
> diff --git a/kernel/hazptr.c b/kernel/hazptr.c
> index a9d3d68a1525..d3d1050d92cf 100644
> --- a/kernel/hazptr.c
> +++ b/kernel/hazptr.c
> @@ -13,6 +13,17 @@
> #include <linux/list.h>
> #include <linux/export.h>
>
> +/*
> + * The current hazard pointer wildcard. Flips between 1UL and 2UL to
> guarantee
> + * hazptr_synchronize forward progress even with a steady stream of readers.
> + * This wildcard value is used by acquire to temporarily tag the per-CPU
> slots.
> + * This also affects the overflow list selection: the current list used by
> + * readers is array[(unsigned long) hazptr_wildcard - 1].
> + */
> +static DEFINE_MUTEX(hazptr_wildcard_lock); /* Protect the wildcard flip. */
> +void *hazptr_wildcard = (void *) 1UL;
> +EXPORT_SYMBOL_GPL(hazptr_wildcard);
> +
> struct hazptr_overflow_list {
> raw_spinlock_t lock; /* Lock protecting overflow list and
> list generation. */
> struct hlist_head head; /* Overflow list head. */
> @@ -28,8 +39,6 @@ struct hazptr_overflow_list {
> * limited to the number of list elements.
> */
> struct hazptr_overflow_list_flip {
> - struct mutex lock; /* Mutex protecting add_idx from
> concurrent updates. */
> - unsigned int add_idx; /* Index of current flip-list to add
> to. */
> struct hazptr_overflow_list array[2];
> };
>
> @@ -38,6 +47,20 @@ static DEFINE_PER_CPU(struct hazptr_overflow_list_flip,
> percpu_overflow_list_fli
> DEFINE_PER_CPU(struct hazptr_percpu_slots, hazptr_percpu_slots);
> EXPORT_PER_CPU_SYMBOL_GPL(hazptr_percpu_slots);
>
> +static
> +void *flip_wildcard(void *wildcard)
> +{
> + return ((unsigned long) wildcard == 1UL) ? (void *) 2UL : (void *) 1UL;
> +}
> +
> +static
> +bool is_wildcard(void *addr)
> +{
> + if ((unsigned long) addr == 1UL || (unsigned long) addr == 2UL)
> + return true;
> + return false;
> +}
> +
> static
> struct hazptr_slot *hazptr_get_free_percpu_slot(struct hazptr_ctx *ctx)
> {
> @@ -72,7 +95,7 @@ void *__hazptr_acquire(struct hazptr_ctx *ctx, void * const
> *addr_p)
> */
> if (unlikely(!slot))
> slot = hazptr_chain_backup_slot(ctx);
> - WRITE_ONCE(slot->addr, HAZPTR_WILDCARD); /* Store B */
> + WRITE_ONCE(slot->addr, READ_ONCE(hazptr_wildcard)); /* Store B */
>
> /* Memory ordering: Store B before Load A. */
> smp_mb();
> @@ -118,7 +141,9 @@ void hazptr_synchronize_overflow_list(struct
> hazptr_overflow_list *overflow_list
> for (;;) {
> void *load_addr =
> smp_load_acquire(&backup_slot->slot.addr); /* Load B */
>
> - if (load_addr != addr && load_addr != HAZPTR_WILDCARD)
> + /* We don't expect wildcards in overflow list. */
> + WARN_ON_ONCE(is_wildcard(load_addr));
> + if (load_addr != addr)
> break;
> raw_spin_unlock_irqrestore(&overflow_list->lock, flags);
> cpu_relax();
> @@ -139,7 +164,7 @@ void hazptr_synchronize_overflow_list(struct
> hazptr_overflow_list *overflow_list
> }
>
> static
> -void hazptr_synchronize_cpu_slots(int cpu, void *addr)
> +void hazptr_synchronize_cpu_slots(int cpu, void *addr, void *scan_wildcard)
> {
> struct hazptr_percpu_slots *percpu_slots =
> per_cpu_ptr(&hazptr_percpu_slots, cpu);
> unsigned int idx;
> @@ -148,7 +173,39 @@ void hazptr_synchronize_cpu_slots(int cpu, void *addr)
> struct hazptr_slot_item *item = &percpu_slots->items[idx];
>
> /* Busy-wait if node is found. */
> - smp_cond_load_acquire(&item->slot.addr, VAL != addr && VAL !=
> HAZPTR_WILDCARD); /* Load B */
> + smp_cond_load_acquire(&item->slot.addr, VAL != addr && VAL !=
> scan_wildcard); /* Load B */
> + }
> +}
> +
> +static
> +void hazptr_scan_period(void *addr, void *scan_wildcard)
> +{
> + unsigned int scan_idx = (unsigned long) scan_wildcard - 1;
> + int cpu;
> +
> + /* Scan all CPUs slots. */
> + for_each_possible_cpu(cpu) {
> + struct hazptr_overflow_list_flip *overflow_list_flip =
> per_cpu_ptr(&percpu_overflow_list_flip, cpu);
> +
> + /*
> + * Scan CPU slots.
> + * Forward progress against recurring wildcards is guaranteed
> + * by scanning for one wildcard while new elements use the
> + * other wildcard value (1UL vs 2UL).
> + * Forward progress against recurring single hazard pointer
> + * values is guaranteed by the fact that a hazard pointer
> + * is not reclaimed nor reused until the scan for that hazard
> + * pointer completes, which prevents a steady flow of readers
> + * to acquire that same hazard pointer value.
> + */
> + hazptr_synchronize_cpu_slots(cpu, addr, scan_wildcard);
> +
> + /*
> + * Scan backup slots in percpu overflow lists.
> + * Forward progress is guaranteed by scanning one list
> + * while new elements are added into the other list.
> + */
> +
> hazptr_synchronize_overflow_list(&overflow_list_flip->array[scan_idx], addr);
> }
> }
>
> @@ -161,7 +218,7 @@ void hazptr_synchronize_cpu_slots(int cpu, void *addr)
> */
> void hazptr_synchronize(void *addr)
> {
> - int cpu;
> + void *scan_wildcard;
>
> /*
> * Busy-wait should only be done from preemptible context.
> @@ -177,33 +234,19 @@ void hazptr_synchronize(void *addr)
> return;
> /* Memory ordering: Store A before Load B. */
> smp_mb();
> - /* Scan all CPUs slots. */
> - for_each_possible_cpu(cpu) {
> - struct hazptr_overflow_list_flip *overflow_list_flip =
> per_cpu_ptr(&percpu_overflow_list_flip, cpu);
> - unsigned int scan_idx;
> -
> - /* Scan CPU slots. */
> - hazptr_synchronize_cpu_slots(cpu, addr);
>
> - /*
> - * Scan backup slots in percpu overflow lists.
> - * Forward progress is guaranteed by scanning one list
> - * while new elements are added into the other list.
> - */
> - guard(mutex)(&overflow_list_flip->lock);
> - scan_idx = overflow_list_flip->add_idx ^ 1;
> -
> hazptr_synchronize_overflow_list(&overflow_list_flip->array[scan_idx], addr);
> - /* Flip current list. */
> - WRITE_ONCE(overflow_list_flip->add_idx, scan_idx);
> -
> hazptr_synchronize_overflow_list(&overflow_list_flip->array[scan_idx ^ 1],
> addr);
> - }
> + guard(mutex)(&hazptr_wildcard_lock);
> + scan_wildcard = flip_wildcard(hazptr_wildcard);
> + hazptr_scan_period(addr, scan_wildcard);
> + WRITE_ONCE(hazptr_wildcard, scan_wildcard); /* Flip the current
> wildcard. */
> + hazptr_scan_period(addr, flip_wildcard(scan_wildcard));
> }
> EXPORT_SYMBOL_GPL(hazptr_synchronize);
>
> struct hazptr_slot *hazptr_chain_backup_slot(struct hazptr_ctx *ctx)
> {
> struct hazptr_overflow_list_flip *overflow_list_flip =
> this_cpu_ptr(&percpu_overflow_list_flip);
> - unsigned int list_idx = READ_ONCE(overflow_list_flip->add_idx);
> + unsigned int list_idx = (unsigned long) READ_ONCE(hazptr_wildcard) - 1;
What if this happens?
{ <hazptr_wildcard == 2UL> }
CPU 0 CPU 1
===== =====
hazptr_acquire(ctx, &gp):
WRITE_ONCE(slot->addr, READ_ONCE(hazptr_wildcard)); /* Store B */
// slot->addr == 2
smp_mb();
addr = READ_ONCE(*addr_p); /* Load A */
// ^ addr == gp == old, i.e not NULL
/* unpublish and wait for reader */
old = gp;
WRITE_ONCE(gp, NULL);
hazptr_synchronize(old):
smp_mb();
guard(mutex)(&hazptr_wildcard_lock);
scan_wildcard =
flip_wildcard(hazptr_wildcard);
// ^ scan_wildcard == 1;
hazptr_scan_period(addr,
scan_wildcard);
// ^ will miss reader on CPU 0
// because its slot->addr == 2
WRITE_ONCE(hazptr_wildcard,
scan_wildcard); /* Flip the current wildcard. */
{ <hazptr_wildcard == 1UL> }
WRITE_ONCE(slot->addr, addr);
hazptr_detach():
hazptr_chain_backup_slot():
list_idx = READ_ONCE(hazptr_wildcard) - 1;
// ^ list_idx == 0
smp_store_release(&slot->addr, NULL);
// ^ clear the per-CPU slot
// flip_wildcard(scan_wildcard) == 2
hazptr_scan_period(addr,
flip_wildcard(scan_wildcard));
// ^ will miss reader on CPU 0
// because it only scans list
// 1.
If I'm not missing anything, then it means a reader can dodge the
hazptr_synchronize() scan, because its per-CPU slot can appear on
wildchard=1 but its backup slot can be on wildcard=2.
Thoughts?
Regards,
Boqun
> struct hazptr_overflow_list *overflow_list =
> &overflow_list_flip->array[list_idx];
> struct hazptr_slot *slot = &ctx->backup_slot.slot;
>
> @@ -233,7 +276,6 @@ void __init hazptr_init(void)
> for_each_possible_cpu(cpu) {
> struct hazptr_overflow_list_flip *overflow_list_flip =
> per_cpu_ptr(&percpu_overflow_list_flip, cpu);
>
> - mutex_init(&overflow_list_flip->lock);
> for (int i = 0; i < 2; i++) {
> raw_spin_lock_init(&overflow_list_flip->array[i].lock);
> INIT_HLIST_HEAD(&overflow_list_flip->array[i].head);
> --
> 2.40.1
>
