Date:	Thu, 7 Apr 2011 14:49:46 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Tony Luck <tony.luck@...el.com>, linux-acpi@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH -v2 3/4] lib, Make gen_pool memory allocator lockless

* Huang Ying (ying.huang@...el.com) wrote:
> This version of the gen_pool memory allocator supports lockless
> operation.
> 
> This makes it safe to use in NMI handlers and other special
> unblockable contexts that could otherwise deadlock on locks.  This is
> implemented by using atomic operations and retries on any conflicts.
> The disadvantage is that there may be livelocks in extreme cases.  For
> better scalability, one gen_pool allocator can be used for each CPU.
> 
> The lockless operation only works if there is enough memory available.
> If new memory is added to the pool a lock has to be still taken.  So
> any user relying on locklessness has to ensure that sufficient memory
> is preallocated.
> 
> The basic atomic operation of this allocator is cmpxchg on long.  On
> architectures that don't have NMI-safe cmpxchg implementation, the
> allocator can NOT be used in NMI handler.  So code uses the allocator
> in NMI handler should depend on CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
> 
> Signed-off-by: Huang Ying <ying.huang@...el.com>
> Reviewed-by: Andi Kleen <ak@...ux.intel.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  include/linux/bitmap.h   |    1 
>  include/linux/genalloc.h |   46 +++++++-
>  lib/bitmap.c             |    2 
>  lib/genalloc.c           |  256 ++++++++++++++++++++++++++++++++++++++---------
>  4 files changed, 250 insertions(+), 55 deletions(-)
> 
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -142,6 +142,7 @@ extern void bitmap_release_region(unsign
>  extern int bitmap_allocate_region(unsigned long *bitmap, int pos, int order);
>  extern void bitmap_copy_le(void *dst, const unsigned long *src, int nbits);
>  
> +#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) % BITS_PER_LONG))
>  #define BITMAP_LAST_WORD_MASK(nbits)					\
>  (									\
>  	((nbits) % BITS_PER_LONG) ?					\
> --- a/include/linux/genalloc.h
> +++ b/include/linux/genalloc.h
> @@ -1,8 +1,28 @@
> +#ifndef GENALLOC_H
> +#define GENALLOC_H
>  /*
> - * Basic general purpose allocator for managing special purpose memory
> - * not managed by the regular kmalloc/kfree interface.
> - * Uses for this includes on-device special memory, uncached memory
> - * etc.
> + * Basic general purpose allocator for managing special purpose
> + * memory, for example, memory that is not managed by the regular
> + * kmalloc/kfree interface.  Uses for this includes on-device special
> + * memory, uncached memory etc.
> + *
> + * It is safe to use the allocator in NMI handlers and other special
> + * unblockable contexts that could otherwise deadlock on locks.  This
> + * is implemented by using atomic operations and retries on any
> + * conflicts.  The disadvantage is that there may be livelocks in
> + * extreme cases.  For better scalability, one allocator can be used
> + * for each CPU.
> + *
> + * The lockless operation only works if there is enough memory
> + * available.  If new memory is added to the pool a lock has to be
> + * still taken.  So any user relying on locklessness has to ensure
> + * that sufficient memory is preallocated.
> + *
> + * The basic atomic operation of this allocator is cmpxchg on long.
> + * On architectures that don't have NMI-safe cmpxchg implementation,
> + * the allocator can NOT be used in NMI handler.  So code uses the
> + * allocator in NMI handler should depend on
> + * CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
>   *
>   * This source code is licensed under the GNU General Public License,
>   * Version 2.  See the file COPYING for more details.
> @@ -13,7 +33,7 @@
>   *  General purpose special memory pool descriptor.
>   */
>  struct gen_pool {
> -	rwlock_t lock;
> +	spinlock_t lock;
>  	struct list_head chunks;	/* list of chunks in this pool */
>  	int min_alloc_order;		/* minimum allocation order */
>  };
> @@ -22,15 +42,29 @@ struct gen_pool {
>   *  General purpose special memory pool chunk descriptor.
>   */
>  struct gen_pool_chunk {
> -	spinlock_t lock;
>  	struct list_head next_chunk;	/* next chunk in pool */
> +	atomic_t avail;
>  	unsigned long start_addr;	/* starting address of memory chunk */
>  	unsigned long end_addr;		/* ending address of memory chunk */
>  	unsigned long bits[0];		/* bitmap for allocating memory chunk */
>  };
>  
> +/**
> + * gen_pool_for_each_chunk - iterate over chunks of generic memory pool
> + * @chunk:	the struct gen_pool_chunk * to use as a loop cursor
> + * @pool:	the generic memory pool
> + *
> + * Not lockless, proper mutual exclusion is needed to use this macro
> + * with other gen_pool function simultaneously.
> + */
> +#define gen_pool_for_each_chunk(chunk, pool)			\
> +	list_for_each_entry_rcu(chunk, &(pool)->chunks, next_chunk)
> +
>  extern struct gen_pool *gen_pool_create(int, int);
>  extern int gen_pool_add(struct gen_pool *, unsigned long, size_t, int);
>  extern void gen_pool_destroy(struct gen_pool *);
>  extern unsigned long gen_pool_alloc(struct gen_pool *, size_t);
>  extern void gen_pool_free(struct gen_pool *, unsigned long, size_t);
> +extern size_t gen_pool_avail(struct gen_pool *);
> +extern size_t gen_pool_size(struct gen_pool *);
> +#endif /* GENALLOC_H */
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -271,8 +271,6 @@ int __bitmap_weight(const unsigned long
>  }
>  EXPORT_SYMBOL(__bitmap_weight);
>  
> -#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) % BITS_PER_LONG))
> -
>  void bitmap_set(unsigned long *map, int start, int nr)
>  {
>  	unsigned long *p = map + BIT_WORD(start);
> --- a/lib/genalloc.c
> +++ b/lib/genalloc.c
> @@ -1,8 +1,33 @@
>  /*
> - * Basic general purpose allocator for managing special purpose memory
> - * not managed by the regular kmalloc/kfree interface.
> - * Uses for this includes on-device special memory, uncached memory
> - * etc.
> + * Basic general purpose allocator for managing special purpose
> + * memory, for example, memory that is not managed by the regular
> + * kmalloc/kfree interface.  Uses for this includes on-device special
> + * memory, uncached memory etc.
> + *
> + * It is safe to use the allocator in NMI handlers and other special
> + * unblockable contexts that could otherwise deadlock on locks.  This
> + * is implemented by using atomic operations and retries on any
> + * conflicts.  The disadvantage is that there may be livelocks in
> + * extreme cases.  For better scalability, one allocator can be used
> + * for each CPU.
> + *
> + * The lockless operation only works if there is enough memory
> + * available.  If new memory is added to the pool a lock has to be
> + * still taken.  So any user relying on locklessness has to ensure
> + * that sufficient memory is preallocated.
> + *
> + * The basic atomic operation of this allocator is cmpxchg on long.
> + * On architectures that don't have NMI-safe cmpxchg implementation,
> + * the allocator can NOT be used in NMI handler.  So code uses the
> + * allocator in NMI handler should depend on
> + * CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG.
> + *
> + * rcu_read_lock and rcu_read_unlock is not used int gen_pool_alloc,
> + * gen_pool_free, gen_pool_avail and gen_pool_size etc, because chunks
> + * are only added into pool, not deleted from pool unless the pool
> + * itself is destroyed.  If chunk will be deleted from pool,
> + * rcu_read_lock and rcu_read_unlock should be uses in these
> + * functions.
>   *
>   * Copyright 2005 (C) Jes Sorensen <jes@...ined-monkey.org>
>   *
> @@ -13,8 +38,109 @@
>  #include <linux/slab.h>
>  #include <linux/module.h>
>  #include <linux/bitmap.h>
> +#include <linux/rculist.h>
> +#include <linux/interrupt.h>
>  #include <linux/genalloc.h>
>  
> +static int set_bits_ll(unsigned long *addr, unsigned long mask_to_set)
> +{
> +	unsigned long val, nval;
> +
> +	nval = *addr;
> +	do {
> +		val = nval;
> +		if (val & mask_to_set)
> +			return -EBUSY;
> +		cpu_relax();
> +	} while ((nval = cmpxchg(addr, val, val | mask_to_set)) != val);

Some architectures have their own atomic set bit already (e.g. intel),
you should probably extend the existing set "bit" to a set "bits"
instead, and use that instead for those, and put the generic
implementation in asm-generic.

> +
> +	return 0;
> +}
> +
> +static int clear_bits_ll(unsigned long *addr, unsigned long mask_to_clear)
> +{
> +	unsigned long val, nval;
> +
> +	nval = *addr;
> +	do {
> +		val = nval;
> +		if ((val & mask_to_clear) != mask_to_clear)
> +			return -EBUSY;
> +		cpu_relax();
> +	} while ((nval = cmpxchg(addr, val, val & ~mask_to_clear)) != val);

Same as above.

> +
> +	return 0;
> +}
> +
> +/*
> + * bitmap_set_ll - set the specified number of bits at the specified position
> + * @map: pointer to a bitmap
> + * @start: a bit position in @map
> + * @nr: number of bits to set
> + *
> + * Set @nr bits start from @start in @map lock-lessly. Several users
> + * can set/clear the same bitmap simultaneously without lock. If two
> + * users set the same bit, one user will return remain bits, otherwise
> + * return 0.
> + */
> +static int bitmap_set_ll(unsigned long *map, int start, int nr)
> +{
> +	unsigned long *p = map + BIT_WORD(start);
> +	const int size = start + nr;
> +	int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
> +	unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);

Ah :) I've had some fun working on bitfield management headers. First
question: how did you test this code ? Shift of "32" being turned to a
no-op on Intel is an example of how some odd cases can creep into this
kind of code. If you are interested, you might want to have a look at my
portable bitfield read/write MIT-licensed header in the Babeltrace
library, file include/babeltrace/bitfield.h
(http://git.efficios.com/?p=babeltrace.git).  It's not using atomic
read/writes, but supports bitfield read/write event across different
endiannesses. I made a testing program for it by providing limit values
and random value, and checking that what is read/written matches. That
helped me find interesting corner-cases.

> +
> +	while (nr - bits_to_set >= 0) {
> +		if (set_bits_ll(p, mask_to_set))
> +			return nr;
> +		nr -= bits_to_set;
> +		bits_to_set = BITS_PER_LONG;
> +		mask_to_set = ~0UL;
> +		p++;
> +	}
> +	if (nr) {
> +		mask_to_set &= BITMAP_LAST_WORD_MASK(size);
> +		if (set_bits_ll(p, mask_to_set))
> +			return nr;
> +	}
> +
> +	return 0;
> +}
> +
> +/*
> + * bitmap_clear_ll - clear the specified number of bits at the specified position
> + * @map: pointer to a bitmap
> + * @start: a bit position in @map
> + * @nr: number of bits to set
> + *
> + * Clear @nr bits start from @start in @map lock-lessly. Several users
> + * can set/clear the same bitmap simultaneously without lock. If two
> + * users clear the same bit, one user will return remain bits,
> + * otherwise return 0.
> + */
> +static int bitmap_clear_ll(unsigned long *map, int start, int nr)
> +{
> +	unsigned long *p = map + BIT_WORD(start);
> +	const int size = start + nr;
> +	int bits_to_clear = BITS_PER_LONG - (start % BITS_PER_LONG);
> +	unsigned long mask_to_clear = BITMAP_FIRST_WORD_MASK(start);
> +
> +	while (nr - bits_to_clear >= 0) {
> +		if (clear_bits_ll(p, mask_to_clear))
> +			return nr;
> +		nr -= bits_to_clear;
> +		bits_to_clear = BITS_PER_LONG;
> +		mask_to_clear = ~0UL;
> +		p++;
> +	}
> +	if (nr) {
> +		mask_to_clear &= BITMAP_LAST_WORD_MASK(size);
> +		if (clear_bits_ll(p, mask_to_clear))
> +			return nr;
> +	}
> +
> +	return 0;
> +}
>  
>  /**
>   * gen_pool_create - create a new special memory pool
> @@ -30,7 +156,7 @@ struct gen_pool *gen_pool_create(int min
>  
>  	pool = kmalloc_node(sizeof(struct gen_pool), GFP_KERNEL, nid);
>  	if (pool != NULL) {
> -		rwlock_init(&pool->lock);
> +		spin_lock_init(&pool->lock);
>  		INIT_LIST_HEAD(&pool->chunks);
>  		pool->min_alloc_order = min_alloc_order;
>  	}
> @@ -58,15 +184,15 @@ int gen_pool_add(struct gen_pool *pool,
>  
>  	chunk = kmalloc_node(nbytes, GFP_KERNEL | __GFP_ZERO, nid);
>  	if (unlikely(chunk == NULL))
> -		return -1;
> +		return -ENOMEM;
>  
> -	spin_lock_init(&chunk->lock);
>  	chunk->start_addr = addr;
>  	chunk->end_addr = addr + size;
> +	atomic_set(&chunk->avail, size);
>  
> -	write_lock(&pool->lock);
> -	list_add(&chunk->next_chunk, &pool->chunks);
> -	write_unlock(&pool->lock);
> +	spin_lock(&pool->lock);
> +	list_add_rcu(&chunk->next_chunk, &pool->chunks);

hrm, where is the list_del_rcu ? Is there anywhere where we have some
call_rcu scheme or synchronize_rcu to handle chunk teardown ?

> +	spin_unlock(&pool->lock);
>  
>  	return 0;
>  }
> @@ -86,7 +212,6 @@ void gen_pool_destroy(struct gen_pool *p
>  	int order = pool->min_alloc_order;
>  	int bit, end_bit;
>  
> -
>  	list_for_each_safe(_chunk, _next_chunk, &pool->chunks) {
>  		chunk = list_entry(_chunk, struct gen_pool_chunk, next_chunk);
>  		list_del(&chunk->next_chunk);
> @@ -108,43 +233,47 @@ EXPORT_SYMBOL(gen_pool_destroy);
>   * @size: number of bytes to allocate from the pool
>   *
>   * Allocate the requested number of bytes from the specified pool.
> - * Uses a first-fit algorithm.
> + * Uses a first-fit algorithm. Can not be used in NMI handler on
> + * architectures without NMI-safe cmpxchg implementation.
>   */
>  unsigned long gen_pool_alloc(struct gen_pool *pool, size_t size)
>  {
> -	struct list_head *_chunk;
>  	struct gen_pool_chunk *chunk;
> -	unsigned long addr, flags;
> +	unsigned long addr;
>  	int order = pool->min_alloc_order;
> -	int nbits, start_bit, end_bit;
> +	int nbits, start_bit = 0, end_bit, remain;
> +
> +#ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
> +	BUG_ON(in_nmi());
> +#endif
>  
>  	if (size == 0)
>  		return 0;
>  
>  	nbits = (size + (1UL << order) - 1) >> order;
> -
> -	read_lock(&pool->lock);
> -	list_for_each(_chunk, &pool->chunks) {
> -		chunk = list_entry(_chunk, struct gen_pool_chunk, next_chunk);

missing rcu_read_lock() ?

> +	list_for_each_entry_rcu(chunk, &pool->chunks, next_chunk) {
> +		if (size > atomic_read(&chunk->avail))
> +			continue;
>  
>  		end_bit = (chunk->end_addr - chunk->start_addr) >> order;
> -
> -		spin_lock_irqsave(&chunk->lock, flags);
> -		start_bit = bitmap_find_next_zero_area(chunk->bits, end_bit, 0,
> -						nbits, 0);
> -		if (start_bit >= end_bit) {
> -			spin_unlock_irqrestore(&chunk->lock, flags);
> +retry:
> +		start_bit = bitmap_find_next_zero_area(chunk->bits, end_bit,
> +						       start_bit, nbits, 0);
> +		if (start_bit >= end_bit)
>  			continue;
> +		remain = bitmap_set_ll(chunk->bits, start_bit, nbits);
> +		if (remain) {
> +			remain = bitmap_clear_ll(chunk->bits, start_bit,
> +						 nbits - remain);
> +			BUG_ON(remain);

maybe add cpu_relax() ? This is a busy loop after all.

> +			goto retry;
>  		}
>  
>  		addr = chunk->start_addr + ((unsigned long)start_bit << order);
> -
> -		bitmap_set(chunk->bits, start_bit, nbits);
> -		spin_unlock_irqrestore(&chunk->lock, flags);
> -		read_unlock(&pool->lock);
> +		size = nbits << order;
> +		atomic_sub(size, &chunk->avail);
>  		return addr;
>  	}
> -	read_unlock(&pool->lock);
>  	return 0;
>  }
>  EXPORT_SYMBOL(gen_pool_alloc);
> @@ -155,33 +284,66 @@ EXPORT_SYMBOL(gen_pool_alloc);
>   * @addr: starting address of memory to free back to pool
>   * @size: size in bytes of memory to free
>   *
> - * Free previously allocated special memory back to the specified pool.
> + * Free previously allocated special memory back to the specified
> + * pool.  Can not be used in NMI handler on architectures without
> + * NMI-safe cmpxchg implementation.
>   */
>  void gen_pool_free(struct gen_pool *pool, unsigned long addr, size_t size)
>  {
> -	struct list_head *_chunk;
>  	struct gen_pool_chunk *chunk;
> -	unsigned long flags;
>  	int order = pool->min_alloc_order;
> -	int bit, nbits;
> -
> -	nbits = (size + (1UL << order) - 1) >> order;
> +	int start_bit, nbits, remain;
>  
> -	read_lock(&pool->lock);
> -	list_for_each(_chunk, &pool->chunks) {
> -		chunk = list_entry(_chunk, struct gen_pool_chunk, next_chunk);
> +#ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
> +	BUG_ON(in_nmi());
> +#endif
>  
> +	nbits = (size + (1UL << order) - 1) >> order;

missing rcu_read_lock ?

> +	list_for_each_entry_rcu(chunk, &pool->chunks, next_chunk) {
>  		if (addr >= chunk->start_addr && addr < chunk->end_addr) {
>  			BUG_ON(addr + size > chunk->end_addr);
> -			spin_lock_irqsave(&chunk->lock, flags);
> -			bit = (addr - chunk->start_addr) >> order;
> -			while (nbits--)
> -				__clear_bit(bit++, chunk->bits);
> -			spin_unlock_irqrestore(&chunk->lock, flags);
> -			break;
> +			start_bit = (addr - chunk->start_addr) >> order;
> +			remain = bitmap_clear_ll(chunk->bits, start_bit, nbits);
> +			BUG_ON(remain);
> +			size = nbits << order;
> +			atomic_add(size, &chunk->avail);
> +			return;
>  		}
>  	}
> -	BUG_ON(nbits > 0);
> -	read_unlock(&pool->lock);
> +	BUG();
>  }
>  EXPORT_SYMBOL(gen_pool_free);
> +
> +/**
> + * gen_pool_avail - get available free space of the pool
> + * @pool: pool to get available free space
> + *
> + * Return available free space of the specified pool.
> + */
> +size_t gen_pool_avail(struct gen_pool *pool)
> +{
> +	struct gen_pool_chunk *chunk;
> +	size_t avail = 0;
> +

rcu_read_lock ?

> +	list_for_each_entry_rcu(chunk, &pool->chunks, next_chunk)
> +		avail += atomic_read(&chunk->avail);
> +	return avail;
> +}
> +EXPORT_SYMBOL_GPL(gen_pool_avail);
> +
> +/**
> + * gen_pool_size - get size in bytes of memory managed by the pool
> + * @pool: pool to get size
> + *
> + * Return size in bytes of memory managed by the pool.
> + */
> +size_t gen_pool_size(struct gen_pool *pool)
> +{
> +	struct gen_pool_chunk *chunk;
> +	size_t size = 0;
> +

rcu_read_lock ?

Thanks,

Mathieu

> +	list_for_each_entry_rcu(chunk, &pool->chunks, next_chunk)
> +		size += chunk->end_addr - chunk->start_addr;
> +	return size;
> +}
> +EXPORT_SYMBOL_GPL(gen_pool_size);

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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