Re: [PATCH 26/28] hazptr: Implement two-phase wildcard scan

[Boqun Feng <boqun@kernel.org>]

On Sun, Sep 20, 2026 at 08:46:55AM -0400, Mathieu Desnoyers wrote:
> On 2026-09-19 09:28, Boqun Feng wrote:
> > On Fri, Sep 18, 2026 at 05:00:54PM -0700, Paul E. McKenney wrote:
> > > From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > 
> > > 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 <mathieu.desnoyers@efficios.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > Cc: Boqun Feng <boqun@kernel.org>
> > > Reviewed-by: Bradley Morgan <include@grrlz.net>
> > > ---
> > >   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.
> 
> The scenario presented here includes a call to hazptr_detach,
> which moves the slot to the backup list, which is handled by

But the slot move happens between the two scans in one
hazptr_synchronize(), so it's moved to the scan list 0 instead of 1,
after we already finished the the scan of list 0, no? That's why the
scan can miss it.

> hazptr_synchronize() _after_ scanning the per-cpu slots
> for address and both wildcard values. So the synchronize
> algorithm on the right column should be completed to show the
> role of the backup slot handling as well.
> 

I don't think I see the enough explanantion here, maybe you can
elaborate more? Especially when the hazptr_detach() happens in-between
these two hazptr_scan_period()?

Regards,
BOqun

> Thanks,
> 
> Mathieu
> 
> 
> > 
> > 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
> > > 
> 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com