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Date:   Mon, 9 Oct 2023 03:04:22 +0800
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To:     "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc:     linux-trace-kernel@vger.kernel.org, davem@davemloft.net,
        anil.s.keshavamurthy@intel.com, naveen.n.rao@linux.ibm.com,
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        akpm@linux-foundation.org, sander@svanheule.net,
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References: <20230905015255.81545-1-wuqiang.matt@bytedance.com>
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 <20230925184212.5a0b6c627a837b50424c4a75@kernel.org>
From:   wuqiang <wuqiang.matt@bytedance.com>
Subject: Re: [PATCH v9 1/5] lib: objpool added: ring-array based lockless MPMC
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On 2023/9/25 17:42, Masami Hiramatsu (Google) wrote:
> Hi Wuqiang,
> 
> On Tue,  5 Sep 2023 09:52:51 +0800
> "wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:
> 
>> The object pool is a scalable implementaion of high performance queue
>> for object allocation and reclamation, such as kretprobe instances.
>>
>> With leveraging percpu ring-array to mitigate the hot spot of memory
>> contention, it could deliver near-linear scalability for high parallel
>> scenarios. The objpool is best suited for following cases:
>> 1) Memory allocation or reclamation are prohibited or too expensive
>> 2) Consumers are of different priorities, such as irqs and threads
>>
>> Limitations:
>> 1) Maximum objects (capacity) is determined during pool initializing
>>     and can't be modified (extended) after objpool creation
>> 2) The memory of objects won't be freed until objpool is finalized
>> 3) Object allocation (pop) may fail after trying all cpu slots
> 
> I made a simplifying patch on this by (mainly) removing ages array.
> I also rename local variable to use more readable names, like slot,
> pool, and obj.
> 
> Here the results which I run the test_objpool.ko.
> 
> Original:
> [   50.500235] Summary of testcases:
> [   50.503139]     duration: 1027135us 	hits:   30628293 	miss:          0 	sync: percpu objpool
> [   50.510416]     duration: 1047977us 	hits:   30453023 	miss:          0 	sync: percpu objpool from vmalloc
> [   50.517421]     duration: 1047098us 	hits:   31145034 	miss:          0 	sync & hrtimer: percpu objpool
> [   50.524671]     duration: 1053233us 	hits:   30919640 	miss:          0 	sync & hrtimer: percpu objpool from vmalloc
> [   50.531382]     duration: 1055822us 	hits:    3407221 	miss:     830219 	sync overrun: percpu objpool
> [   50.538135]     duration: 1055998us 	hits:    3404624 	miss:     854160 	sync overrun: percpu objpool from vmalloc
> [   50.546686]     duration: 1046104us 	hits:   19464798 	miss:          0 	async: percpu objpool
> [   50.552989]     duration: 1033054us 	hits:   18957836 	miss:          0 	async: percpu objpool from vmalloc
> [   50.560289]     duration: 1041778us 	hits:   33806868 	miss:          0 	async & hrtimer: percpu objpool
> [   50.567425]     duration: 1048901us 	hits:   34211860 	miss:          0 	async & hrtimer: percpu objpool from vmalloc
> 
> Simplified:
> [   48.393236] Summary of testcases:
> [   48.395384]     duration: 1013002us 	hits:   29661448 	miss:          0 	sync: percpu objpool
> [   48.400351]     duration: 1057231us 	hits:   31035578 	miss:          0 	sync: percpu objpool from vmalloc
> [   48.405660]     duration: 1043196us 	hits:   30546652 	miss:          0 	sync & hrtimer: percpu objpool
> [   48.411216]     duration: 1047128us 	hits:   30601306 	miss:          0 	sync & hrtimer: percpu objpool from vmalloc
> [   48.417231]     duration: 1051695us 	hits:    3468287 	miss:     892881 	sync overrun: percpu objpool
> [   48.422405]     duration: 1054003us 	hits:    3549491 	miss:     898141 	sync overrun: percpu objpool from vmalloc
> [   48.428425]     duration: 1052946us 	hits:   19005228 	miss:          0 	async: percpu objpool
> [   48.433597]     duration: 1051757us 	hits:   19670866 	miss:          0 	async: percpu objpool from vmalloc
> [   48.439280]     duration: 1042951us 	hits:   37135332 	miss:          0 	async & hrtimer: percpu objpool
> [   48.445085]     duration: 1029803us 	hits:   37093248 	miss:          0 	async & hrtimer: percpu objpool from vmalloc
> 
> Can you test it too?

Sure, I'll do some testings locally. Also do a performance comparsion with
'entries' managed in objpool_head.

> 
> Thanks,

Thank you for your time, I appreciate it.

> 
>  From f1f442ff653e329839e5452b8b88463a80a12ff3 Mon Sep 17 00:00:00 2001
> From: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
> Date: Mon, 25 Sep 2023 16:07:12 +0900
> Subject: [PATCH] objpool: Simplify objpool by removing ages array
> 
> Simplify the objpool code by removing ages array from per-cpu slot.
> It chooses enough big capacity (which is a rounded up power of 2 value
> of nr_objects + 1) for the entries array, the tail never catch up to
> the head in per-cpu slot. Thus tail == head means the slot is empty.
> 
> This also uses consistent local variable names for pool, slot and obj.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>   include/linux/objpool.h |  61 ++++----
>   lib/objpool.c           | 310 ++++++++++++++++------------------------
>   2 files changed, 147 insertions(+), 224 deletions(-)
> 
> diff --git a/include/linux/objpool.h b/include/linux/objpool.h
> index 33c832216b98..ecd5ecaffcd3 100644
> --- a/include/linux/objpool.h
> +++ b/include/linux/objpool.h
> @@ -38,33 +38,23 @@
>    * struct objpool_slot - percpu ring array of objpool
>    * @head: head of the local ring array (to retrieve at)
>    * @tail: tail of the local ring array (to append at)
> - * @bits: log2 of capacity (for bitwise operations)
> - * @mask: capacity - 1
> + * @mask: capacity of entries - 1
> + * @entries: object entries on this slot.
>    *
>    * Represents a cpu-local array-based ring buffer, its size is specialized
>    * during initialization of object pool. The percpu objpool slot is to be
>    * allocated from local memory for NUMA system, and to be kept compact in
> - * continuous memory: ages[] is stored just after the body of objpool_slot,
> - * and then entries[]. ages[] describes revision of each item, solely used
> - * to avoid ABA; entries[] contains pointers of the actual objects
> - *
> - * Layout of objpool_slot in memory:
> - *
> - * 64bit:
> - *        4      8      12     16        32                 64
> - * | head | tail | bits | mask | ages[4] | ents[4]: (8 * 4) | objects
> - *
> - * 32bit:
> - *        4      8      12     16        32        48       64
> - * | head | tail | bits | mask | ages[4] | ents[4] | unused | objects
> + * continuous memory: CPU assigned number of objects are stored just after
> + * the body of objpool_slot.
>    *
>    */
>   struct objpool_slot {
> -	uint32_t                head;
> -	uint32_t                tail;
> -	uint32_t                bits;
> -	uint32_t                mask;
> -} __packed;
> +	uint32_t	head;
> +	uint32_t	tail;
> +	uint32_t	mask;
> +	uint32_t	dummyr;
> +	void *		entries[];
> +};
>   
>   struct objpool_head;
>   
> @@ -82,12 +72,11 @@ typedef int (*objpool_fini_cb)(struct objpool_head *head, void *context);
>    * @obj_size:   object & element size
>    * @nr_objs:    total objs (to be pre-allocated)
>    * @nr_cpus:    nr_cpu_ids
> - * @capacity:   max objects per cpuslot
> + * @capacity:   max objects on percpu slot
>    * @gfp:        gfp flags for kmalloc & vmalloc
>    * @ref:        refcount for objpool
>    * @flags:      flags for objpool management
>    * @cpu_slots:  array of percpu slots
> - * @slot_sizes:	size in bytes of slots
>    * @release:    resource cleanup callback
>    * @context:    caller-provided context
>    */
> @@ -100,7 +89,6 @@ struct objpool_head {
>   	refcount_t              ref;
>   	unsigned long           flags;
>   	struct objpool_slot   **cpu_slots;
> -	int                    *slot_sizes;
>   	objpool_fini_cb         release;
>   	void                   *context;
>   };
> @@ -108,9 +96,12 @@ struct objpool_head {
>   #define OBJPOOL_FROM_VMALLOC	(0x800000000)	/* objpool allocated from vmalloc area */
>   #define OBJPOOL_HAVE_OBJECTS	(0x400000000)	/* objects allocated along with objpool */
>   
> +#define OBJPOOL_NR_OBJECT_MAX	(1 << 24)
> +#define OBJPOOL_OBJECT_SIZE_MAX	(1 << 16)
> +
>   /**
>    * objpool_init() - initialize objpool and pre-allocated objects
> - * @head:    the object pool to be initialized, declared by caller
> + * @pool:    the object pool to be initialized, declared by caller
>    * @nr_objs: total objects to be pre-allocated by this object pool
>    * @object_size: size of an object (should be > 0)
>    * @gfp:     flags for memory allocation (via kmalloc or vmalloc)
> @@ -128,47 +119,47 @@ struct objpool_head {
>    * pop (object allocation) or do clearance before each push (object
>    * reclamation).
>    */
> -int objpool_init(struct objpool_head *head, int nr_objs, int object_size,
> +int objpool_init(struct objpool_head *pool, int nr_objs, int object_size,
>   		 gfp_t gfp, void *context, objpool_init_obj_cb objinit,
>   		 objpool_fini_cb release);
>   
>   /**
>    * objpool_pop() - allocate an object from objpool
> - * @head: object pool
> + * @pool: object pool
>    *
>    * return value: object ptr or NULL if failed
>    */
> -void *objpool_pop(struct objpool_head *head);
> +void *objpool_pop(struct objpool_head *pool);
>   
>   /**
>    * objpool_push() - reclaim the object and return back to objpool
>    * @obj:  object ptr to be pushed to objpool
> - * @head: object pool
> + * @pool: object pool
>    *
>    * return: 0 or error code (it fails only when user tries to push
>    * the same object multiple times or wrong "objects" into objpool)
>    */
> -int objpool_push(void *obj, struct objpool_head *head);
> +int objpool_push(void *obj, struct objpool_head *pool);
>   
>   /**
>    * objpool_drop() - discard the object and deref objpool
>    * @obj:  object ptr to be discarded
> - * @head: object pool
> + * @pool: object pool
>    *
>    * return: 0 if objpool was released or error code
>    */
> -int objpool_drop(void *obj, struct objpool_head *head);
> +int objpool_drop(void *obj, struct objpool_head *pool);
>   
>   /**
>    * objpool_free() - release objpool forcely (all objects to be freed)
> - * @head: object pool to be released
> + * @pool: object pool to be released
>    */
> -void objpool_free(struct objpool_head *head);
> +void objpool_free(struct objpool_head *pool);
>   
>   /**
>    * objpool_fini() - deref object pool (also releasing unused objects)
> - * @head: object pool to be dereferenced
> + * @pool: object pool to be dereferenced
>    */
> -void objpool_fini(struct objpool_head *head);
> +void objpool_fini(struct objpool_head *pool);
>   
>   #endif /* _LINUX_OBJPOOL_H */
> diff --git a/lib/objpool.c b/lib/objpool.c
> index 22e752371820..f8e8f70d7253 100644
> --- a/lib/objpool.c
> +++ b/lib/objpool.c
> @@ -15,104 +15,55 @@
>    * Copyright: wuqiang.matt@bytedance.com
>    */
>   
> -#define SLOT_AGES(s) ((uint32_t *)((char *)(s) + sizeof(struct objpool_slot)))
> -#define SLOT_ENTS(s) ((void **)((char *)(s) + sizeof(struct objpool_slot) + \
> -			(sizeof(uint32_t) << (s)->bits)))
> -#define SLOT_OBJS(s) ((void *)((char *)(s) + sizeof(struct objpool_slot) + \
> -			((sizeof(uint32_t) + sizeof(void *)) << (s)->bits)))
> -#define SLOT_CORE(n) cpumask_nth((n) % num_possible_cpus(), cpu_possible_mask)
> -
> -/* compute the suitable num of objects to be managed per slot */
> -static int objpool_nobjs(int size)
> -{
> -	return rounddown_pow_of_two((size - sizeof(struct objpool_slot)) /
> -			(sizeof(uint32_t) + sizeof(void *)));
> -}
> -
>   /* allocate and initialize percpu slots */
>   static int
> -objpool_init_percpu_slots(struct objpool_head *head, int nobjs,
> -			void *context, objpool_init_obj_cb objinit)
> +objpool_init_percpu_slots(struct objpool_head *pool, void *context,
> +			  objpool_init_obj_cb objinit)
>   {
> -	int i, j, n, size, objsz, cpu = 0, nents = head->capacity;
> -
> -	/* aligned object size by sizeof(void *) */
> -	objsz = ALIGN(head->obj_size, sizeof(void *));
> -	/* shall we allocate objects along with percpu-slot */
> -	if (objsz)
> -		head->flags |= OBJPOOL_HAVE_OBJECTS;
> -
> -	/* vmalloc is used in default to allocate percpu-slots */
> -	if (!(head->gfp & GFP_ATOMIC))
> -		head->flags |= OBJPOOL_FROM_VMALLOC;
> -
> -	for (i = 0; i < head->nr_cpus; i++) {
> -		struct objpool_slot *os;
> +	int i, j, n, size, slot_size, cpu_count = 0;
> +	struct objpool_slot *slot;
>   
> +	for (i = 0; i < pool->nr_cpus; i++) {
>   		/* skip the cpus which could never be present */
>   		if (!cpu_possible(i))
>   			continue;
>   
>   		/* compute how many objects to be managed by this slot */
> -		n = nobjs / num_possible_cpus();
> -		if (cpu < (nobjs % num_possible_cpus()))
> +		n = pool->nr_objs / num_possible_cpus();
> +		if (cpu_count < (pool->nr_objs % num_possible_cpus()))
>   			n++;
> -		size = sizeof(struct objpool_slot) + sizeof(void *) * nents +
> -		       sizeof(uint32_t) * nents + objsz * n;
> +		cpu_count++;
> +
> +		slot_size = struct_size(slot, entries, pool->capacity);
> +		size = slot_size + pool->obj_size * n;
>   
>   		/*
>   		 * here we allocate percpu-slot & objects together in a single
> -		 * allocation, taking advantage of warm caches and TLB hits as
> -		 * vmalloc always aligns the request size to pages
> +		 * allocation, taking advantage on NUMA.
>   		 */
> -		if (head->flags & OBJPOOL_FROM_VMALLOC)
> -			os = __vmalloc_node(size, sizeof(void *), head->gfp,
> +		if (pool->flags & OBJPOOL_FROM_VMALLOC)
> +			slot = __vmalloc_node(size, sizeof(void *), pool->gfp,
>   				cpu_to_node(i), __builtin_return_address(0));
>   		else
> -			os = kmalloc_node(size, head->gfp, cpu_to_node(i));
> -		if (!os)
> +			slot = kmalloc_node(size, pool->gfp, cpu_to_node(i));
> +		if (!slot)
>   			return -ENOMEM;
>   
>   		/* initialize percpu slot for the i-th slot */
> -		memset(os, 0, size);
> -		os->bits = ilog2(head->capacity);
> -		os->mask = head->capacity - 1;
> -		head->cpu_slots[i] = os;
> -		head->slot_sizes[i] = size;
> -		cpu = cpu + 1;
> -
> -		/*
> -		 * manually set head & tail to avoid possible conflict:
> -		 * We assume that the head item is ready for retrieval
> -		 * iff head is equal to ages[head & mask]. but ages is
> -		 * initialized as 0, so in view of the caller of pop(),
> -		 * the 1st item (0th) is always ready, but the reality
> -		 * could be: push() is stalled before the final update,
> -		 * thus the item being inserted will be lost forever
> -		 */
> -		os->head = os->tail = head->capacity;
> -
> -		if (!objsz)
> -			continue;
> +		memset(slot, 0, size);
> +		slot->mask = pool->capacity - 1;
> +		pool->cpu_slots[i] = slot;
>   
>   		for (j = 0; j < n; j++) {
> -			uint32_t *ages = SLOT_AGES(os);
> -			void **ents = SLOT_ENTS(os);
> -			void *obj = SLOT_OBJS(os) + j * objsz;
> -			uint32_t ie = os->tail & os->mask;
> +			void *obj = (void *)slot + slot_size + pool->obj_size * j;
>   
> -			/* perform object initialization */
>   			if (objinit) {
>   				int rc = objinit(obj, context);
>   				if (rc)
>   					return rc;
>   			}
> -
> -			/* add obj into the ring array */
> -			ents[ie] = obj;
> -			ages[ie] = os->tail;
> -			os->tail++;
> -			head->nr_objs++;
> +			slot->entries[j] = obj;
> +			slot->tail++;
>   		}
>   	}
>   
> @@ -120,164 +71,135 @@ objpool_init_percpu_slots(struct objpool_head *head, int nobjs,
>   }
>   
>   /* cleanup all percpu slots of the object pool */
> -static void objpool_fini_percpu_slots(struct objpool_head *head)
> +static void objpool_fini_percpu_slots(struct objpool_head *pool)
>   {
>   	int i;
>   
> -	if (!head->cpu_slots)
> +	if (!pool->cpu_slots)
>   		return;
>   
> -	for (i = 0; i < head->nr_cpus; i++) {
> -		if (!head->cpu_slots[i])
> -			continue;
> -		if (head->flags & OBJPOOL_FROM_VMALLOC)
> -			vfree(head->cpu_slots[i]);
> -		else
> -			kfree(head->cpu_slots[i]);
> -	}
> -	kfree(head->cpu_slots);
> -	head->cpu_slots = NULL;
> -	head->slot_sizes = NULL;
> +	for (i = 0; i < pool->nr_cpus; i++)
> +		kvfree(pool->cpu_slots[i]);
> +	kfree(pool->cpu_slots);
>   }
>   
>   /* initialize object pool and pre-allocate objects */
> -int objpool_init(struct objpool_head *head, int nr_objs, int object_size,
> +int objpool_init(struct objpool_head *pool, int nr_objs, int object_size,
>   		gfp_t gfp, void *context, objpool_init_obj_cb objinit,
>   		objpool_fini_cb release)
>   {
>   	int nents, rc;
>   
>   	/* check input parameters */
> -	if (nr_objs <= 0 || object_size <= 0)
> +	if (nr_objs <= 0 || object_size <= 0 ||
> +	    nr_objs > OBJPOOL_NR_OBJECT_MAX || object_size > OBJPOOL_OBJECT_SIZE_MAX)
> +		return -EINVAL;
> +
> +	/* Align up to unsigned long size */
> +	object_size = ALIGN(object_size, sizeof(unsigned long));
> +
> +	/*
> +	 * To avoid filling up the entries array in the per-cpu slot,
> +	 * use the power of 2 which is more than N + 1. Thus, the tail never
> +	 * catch up to the pool, and able to use pool/tail as the sequencial
> +	 * number.
> +	 */
> +	nents = roundup_pow_of_two(nr_objs + 1);
> +	if (!nents)
>   		return -EINVAL;
>   
> -	/* calculate percpu slot size (rounded to pow of 2) */
> -	nents = max_t(int, roundup_pow_of_two(nr_objs),
> -			objpool_nobjs(L1_CACHE_BYTES));
> -
> -	/* initialize objpool head */
> -	memset(head, 0, sizeof(struct objpool_head));
> -	head->nr_cpus = nr_cpu_ids;
> -	head->obj_size = object_size;
> -	head->capacity = nents;
> -	head->gfp = gfp & ~__GFP_ZERO;
> -	head->context = context;
> -	head->release = release;
> -
> -	/* allocate array for percpu slots */
> -	head->cpu_slots = kzalloc(head->nr_cpus * sizeof(void *) +
> -			       head->nr_cpus * sizeof(int), head->gfp);
> -	if (!head->cpu_slots)
> +	/* initialize objpool pool */
> +	memset(pool, 0, sizeof(struct objpool_head));
> +	pool->nr_cpus = nr_cpu_ids;
> +	pool->obj_size = object_size;
> +	pool->nr_objs = nr_objs;
> +	/* just prevent to fullfill the per-cpu ring array */
> +	pool->capacity = nents;
> +	pool->gfp = gfp & ~__GFP_ZERO;
> +	pool->context = context;
> +	pool->release = release;
> +	/* vmalloc is used in default to allocate percpu-slots */
> +	if (!(pool->gfp & GFP_ATOMIC))
> +		pool->flags |= OBJPOOL_FROM_VMALLOC;
> +
> +	pool->cpu_slots = kzalloc(pool->nr_cpus * sizeof(void *), pool->gfp);
> +	if (!pool->cpu_slots)
>   		return -ENOMEM;
> -	head->slot_sizes = (int *)&head->cpu_slots[head->nr_cpus];
>   
>   	/* initialize per-cpu slots */
> -	rc = objpool_init_percpu_slots(head, nr_objs, context, objinit);
> +	rc = objpool_init_percpu_slots(pool, context, objinit);
>   	if (rc)
> -		objpool_fini_percpu_slots(head);
> +		objpool_fini_percpu_slots(pool);
>   	else
> -		refcount_set(&head->ref, nr_objs + 1);
> +		refcount_set(&pool->ref, nr_objs + 1);
>   
>   	return rc;
>   }
>   EXPORT_SYMBOL_GPL(objpool_init);
>   
>   /* adding object to slot, abort if the slot was already full */
> -static inline int objpool_try_add_slot(void *obj, struct objpool_slot *os)
> +static inline int objpool_try_add_slot(void *obj, struct objpool_head *pool, int cpu)
>   {
> -	uint32_t *ages = SLOT_AGES(os);
> -	void **ents = SLOT_ENTS(os);
> -	uint32_t head, tail;
> +	struct objpool_slot *slot = pool->cpu_slots[cpu];
> +	uint32_t tail, next;
>   
>   	do {
> -		/* perform memory loading for both head and tail */
> -		head = READ_ONCE(os->head);
> -		tail = READ_ONCE(os->tail);
> -		/* just abort if slot is full */
> -		if (tail - head > os->mask)
> -			return -ENOENT;
> -		/* try to extend tail by 1 using CAS to avoid races */
> -		if (try_cmpxchg_acquire(&os->tail, &tail, tail + 1))
> -			break;
> -	} while (1);
> +		uint32_t head = READ_ONCE(slot->head);
>   
> -	/* the tail-th of slot is reserved for the given obj */
> -	WRITE_ONCE(ents[tail & os->mask], obj);
> -	/* update epoch id to make this object available for pop() */
> -	smp_store_release(&ages[tail & os->mask], tail);
> +		tail = READ_ONCE(slot->tail);
> +		next = tail + 1;
> +
> +		/* This must never happen because capacity >= N + 1 */
> +		if (WARN_ON_ONCE((next > head && next - head > pool->nr_objs) ||
> +				 (next < head && next > head + pool->nr_objs)))
> +			return -EINVAL;
> +
> +	} while (!try_cmpxchg_acquire(&slot->tail, &tail, next));
> +
> +	WRITE_ONCE(slot->entries[tail & slot->mask], obj);
>   	return 0;
>   }
>   
>   /* reclaim an object to object pool */
> -int objpool_push(void *obj, struct objpool_head *oh)
> +int objpool_push(void *obj, struct objpool_head *pool)
>   {
>   	unsigned long flags;
> -	int cpu, rc;
> +	int rc;
>   
>   	/* disable local irq to avoid preemption & interruption */
>   	raw_local_irq_save(flags);
> -	cpu = raw_smp_processor_id();
> -	do {
> -		rc = objpool_try_add_slot(obj, oh->cpu_slots[cpu]);
> -		if (!rc)
> -			break;
> -		cpu = cpumask_next_wrap(cpu, cpu_possible_mask, -1, 1);
> -	} while (1);
> +	rc = objpool_try_add_slot(obj, pool, raw_smp_processor_id());
>   	raw_local_irq_restore(flags);
>   
>   	return rc;
>   }
>   EXPORT_SYMBOL_GPL(objpool_push);
>   
> -/* drop the allocated object, rather reclaim it to objpool */
> -int objpool_drop(void *obj, struct objpool_head *head)
> -{
> -	if (!obj || !head)
> -		return -EINVAL;
> -
> -	if (refcount_dec_and_test(&head->ref)) {
> -		objpool_free(head);
> -		return 0;
> -	}
> -
> -	return -EAGAIN;
> -}
> -EXPORT_SYMBOL_GPL(objpool_drop);
> -
>   /* try to retrieve object from slot */
> -static inline void *objpool_try_get_slot(struct objpool_slot *os)
> +static inline void *objpool_try_get_slot(struct objpool_slot *slot)
>   {
> -	uint32_t *ages = SLOT_AGES(os);
> -	void **ents = SLOT_ENTS(os);
>   	/* do memory load of head to local head */
> -	uint32_t head = smp_load_acquire(&os->head);
> +	uint32_t head = smp_load_acquire(&slot->head);
>   
>   	/* loop if slot isn't empty */
> -	while (head != READ_ONCE(os->tail)) {
> -		uint32_t id = head & os->mask, prev = head;
> +	while (head != READ_ONCE(slot->tail)) {
>   
>   		/* do prefetching of object ents */
> -		prefetch(&ents[id]);
> -
> -		/* check whether this item was ready for retrieval */
> -		if (smp_load_acquire(&ages[id]) == head) {
> -			/* node must have been udpated by push() */
> -			void *node = READ_ONCE(ents[id]);
> -			/* commit and move forward head of the slot */
> -			if (try_cmpxchg_release(&os->head, &head, head + 1))
> -				return node;
> -			/* head was already updated by others */
> -		}
> +		prefetch(&slot->entries[head & slot->mask]);
> +
> +		/* commit and move forward head of the slot */
> +		if (try_cmpxchg_release(&slot->head, &head, head + 1))
> +			/*
> +			 * TBD: check overwrap the tail/head counter and warn
> +			 * if it is broken. But this happens only if this
> +			 * process slows down a lot and another CPU updates
> +			 * the haed/tail just 2^32 + 1 times, and this slot
> +			 * is empty.
> +			 */
> +			return slot->entries[head & slot->mask];
>   
>   		/* re-load head from memory and continue trying */
> -		head = READ_ONCE(os->head);
> -		/*
> -		 * head stays unchanged, so it's very likely there's an
> -		 * ongoing push() on other cpu nodes but yet not update
> -		 * ages[] to mark it's completion
> -		 */
> -		if (head == prev)
> -			break;
> +		head = READ_ONCE(slot->head);
>   	}
>   
>   	return NULL;
> @@ -307,32 +229,42 @@ void *objpool_pop(struct objpool_head *head)
>   EXPORT_SYMBOL_GPL(objpool_pop);
>   
>   /* release whole objpool forcely */
> -void objpool_free(struct objpool_head *head)
> +void objpool_free(struct objpool_head *pool)
>   {
> -	if (!head->cpu_slots)
> +	if (!pool->cpu_slots)
>   		return;
>   
>   	/* release percpu slots */
> -	objpool_fini_percpu_slots(head);
> +	objpool_fini_percpu_slots(pool);
>   
>   	/* call user's cleanup callback if provided */
> -	if (head->release)
> -		head->release(head, head->context);
> +	if (pool->release)
> +		pool->release(pool, pool->context);
>   }
>   EXPORT_SYMBOL_GPL(objpool_free);
>   
> -/* drop unused objects and defref objpool for releasing */
> -void objpool_fini(struct objpool_head *head)
> +/* drop the allocated object, rather reclaim it to objpool */
> +int objpool_drop(void *obj, struct objpool_head *pool)
>   {
> -	void *nod;
> +	if (!obj || !pool)
> +		return -EINVAL;
>   
> -	do {
> -		/* grab object from objpool and drop it */
> -		nod = objpool_pop(head);
> +	if (refcount_dec_and_test(&pool->ref)) {
> +		objpool_free(pool);
> +		return 0;
> +	}
> +
> +	return -EAGAIN;
> +}
> +EXPORT_SYMBOL_GPL(objpool_drop);
> +
> +/* drop unused objects and defref objpool for releasing */
> +void objpool_fini(struct objpool_head *pool)
> +{
> +	void *obj;
>   
> -		/* drop the extra ref of objpool */
> -		if (refcount_dec_and_test(&head->ref))
> -			objpool_free(head);
> -	} while (nod);
> +	/* grab object from objpool and drop it */
> +	while ((obj = objpool_pop(pool)))
> +		objpool_drop(obj, pool);
>   }
>   EXPORT_SYMBOL_GPL(objpool_fini);