Date:	Tue, 08 Jun 2010 21:01:47 +0200
From:	Stefani Seibold <stefani@...bold.net>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] kfifo writer side lock-less support

Hi Huang,

it would be great if you could post an example code how to use your
code.

Stefani

Am Dienstag, den 08.06.2010, 14:45 +0800 schrieb Huang Ying:
> This patch add lock-less support for kfifo writer side amongst
> different contexts on one CPU, such as NMI, IRQ, soft_irq, process,
> etc. This makes kfifo can be used to implement per-CPU lock-less data
> structure.
> 
> The different contexts on one CPU have some kind of preemption
> priority as follow:
> 
> process -> soft_irq -> IRQ -> NMI
> 
> Where preemption priority increases from left to right. That is, the
> right side context can preempt left side context, but not in the
> reverse direction, that means the right side context will run to the
> end before return to the left side context. The lock-less algorithm
> used in the patch take advantage of this.
> 
> The algorithm works in reserve/commit style, where "reserve" only
> allocate the space, while "commit" really makes the data into buffer,
> data is prepared in between. cmpxchg is used in "reserve", this
> guarantees different spaces will be allocated for different
> contexts. Only the "commit" in lowest context will commit all
> allocated spaces. Because all contexts preempting lowest context
> between "reserve" and "commit" will run to the end, all data put into
> buffer are valid.
> 
> Some idea of the lock-less algorithm in the patch comes from Steven
> Rostedt's trace ring buffer implementation.
> 
> The new xxx_ptr interface and kfifo_iter makes it possible to
> write/read content of kfifo in-place in addition to copying out/in.
> 
> Signed-off-by: Huang Ying <ying.huang@...el.com>
> ---
>  include/linux/kfifo.h |   87 +++++++++++++++++-
>  kernel/kfifo.c        |  231 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 305 insertions(+), 13 deletions(-)
> 
> --- a/include/linux/kfifo.h
> +++ b/include/linux/kfifo.h
> @@ -49,6 +49,7 @@ struct kfifo {
>  	unsigned int size;	/* the size of the allocated buffer */
>  	unsigned int in;	/* data is added at offset (in % size) */
>  	unsigned int out;	/* data is extracted from off. (out % size) */
> +	unsigned int reserve;	/* space is reserved at off (reserve % size) */
>  };
>  
>  /*
> @@ -60,6 +61,7 @@ struct kfifo {
>  	(struct kfifo) { \
>  		.size	= s, \
>  		.in	= 0, \
> +		.reserve = 0, \
>  		.out	= 0, \
>  		.buffer = b \
>  	}
> @@ -132,7 +134,7 @@ static inline bool kfifo_initialized(str
>   */
>  static inline void kfifo_reset(struct kfifo *fifo)
>  {
> -	fifo->in = fifo->out = 0;
> +	fifo->reserve = fifo->in = fifo->out = 0;
>  }
>  
>  /**
> @@ -167,6 +169,18 @@ static inline unsigned int kfifo_len(str
>  	return fifo->in - out;
>  }
>  
> +/*
> + * returns the number of used bytes (including reserved) in the FIFO
> + */
> +static inline unsigned int __kfifo_used(struct kfifo *fifo)
> +{
> +	register unsigned int	out;
> +
> +	out = fifo->out;
> +	smp_rmb();
> +	return fifo->reserve - out;
> +}
> +
>  /**
>   * kfifo_is_empty - returns true if the fifo is empty
>   * @fifo: the fifo to be used.
> @@ -182,7 +196,7 @@ static inline __must_check int kfifo_is_
>   */
>  static inline __must_check int kfifo_is_full(struct kfifo *fifo)
>  {
> -	return kfifo_len(fifo) == kfifo_size(fifo);
> +	return __kfifo_used(fifo) == kfifo_size(fifo);
>  }
>  
>  /**
> @@ -191,7 +205,7 @@ static inline __must_check int kfifo_is_
>   */
>  static inline __must_check unsigned int kfifo_avail(struct kfifo *fifo)
>  {
> -	return kfifo_size(fifo) - kfifo_len(fifo);
> +	return kfifo_size(fifo) - __kfifo_used(fifo);
>  }
>  
>  /**
> @@ -269,6 +283,7 @@ static inline void __kfifo_add_out(struc
>  static inline void __kfifo_add_in(struct kfifo *fifo,
>  				unsigned int off)
>  {
> +	fifo->reserve += off;
>  	smp_wmb();
>  	fifo->in += off;
>  }
> @@ -283,6 +298,15 @@ static inline unsigned int __kfifo_off(s
>  }
>  
>  /*
> + * __kfifo_ptr internal helper function to get pointer at the position
> + * for in-place accessing
> + */
> +static inline void *__kfifo_ptr(struct kfifo *fifo, unsigned int pos)
> +{
> +	return fifo->buffer + __kfifo_off(fifo, pos);
> +}
> +
> +/*
>   * __kfifo_peek_n internal helper function for determinate the length of
>   * the next record in the fifo
>   */
> @@ -607,9 +631,64 @@ static inline void kfifo_skip_rec(struct
>  static inline __must_check unsigned int kfifo_avail_rec(struct kfifo *fifo,
>  	unsigned int recsize)
>  {
> -	unsigned int l = kfifo_size(fifo) - kfifo_len(fifo);
> +	unsigned int l = kfifo_size(fifo) - __kfifo_used(fifo);
>  
>  	return (l > recsize) ? l - recsize : 0;
>  }
>  
> +extern __must_check int kfifo_reserve(struct kfifo *fifo,
> +	unsigned int len, unsigned int *ppos);
> +extern void kfifo_commit(struct kfifo *fifo, unsigned int pos);
> +extern void kfifo_in_data(struct kfifo *fifo, const void *from,
> +	unsigned int len, unsigned int off);
> +extern unsigned int kfifo_ll_in(struct kfifo *fifo, const void *from,
> +	unsigned int len);
> +extern __must_check void *kfifo_reserve_continuous_ptr(struct kfifo *fifo,
> +	unsigned int *len);
> +extern void kfifo_commit_ptr(struct kfifo *fifo, void *ptr);
> +
> +struct kfifo_iter {
> +	struct kfifo *fifo;
> +	unsigned int pos;
> +};
> +
> +/**
> + * kfifo_iter_init - initialize a FIFO iterator
> + * @iter: the iterator to be initialized
> + * @fifo: the fifo to be accessed
> + *
> + */
> +static inline void kfifo_iter_init(struct kfifo_iter *iter, struct kfifo *fifo)
> +{
> +	iter->fifo = fifo;
> +	iter->pos = fifo->out;
> +}
> +
> +/**
> + * kfifo_iter_advance - advances the position of iterator
> + * @iter: the iterator to be used
> + * @adv: the bytes to advance
> + *
> + * This function advances the position of iterator by @adv bytes,
> + * usually goes to the position of the next record.
> + */
> +static inline void kfifo_iter_advance(struct kfifo_iter *iter, unsigned int adv)
> +{
> +	iter->pos += adv;
> +}
> +
> +/**
> + * kfifo_iter_get_ptr - gets the pointer to the current position of iterator
> + * @iter: the iterator to be used
> + *
> + * This function returns the pointer to the current position of
> + * iterator. If the iterator is at the end of FIFO, NULL is
> + * returned. This is used to access the data/records in FIFO in-place.
> + */
> +static inline void *kfifo_iter_get_ptr(struct kfifo_iter *iter)
> +{
> +	if (iter->pos == iter->fifo->in)
> +		return NULL;
> +	return __kfifo_ptr(iter->fifo, iter->pos);
> +}
>  #endif
> --- a/kernel/kfifo.c
> +++ b/kernel/kfifo.c
> @@ -116,8 +116,18 @@ void kfifo_skip(struct kfifo *fifo, unsi
>  }
>  EXPORT_SYMBOL(kfifo_skip);
>  
> -static inline void __kfifo_in_data(struct kfifo *fifo,
> -		const void *from, unsigned int len, unsigned int off)
> +/**
> + * kfifo_in_data - copies some data into FIFO
> + * @fifo: the fifo to be used.
> + * @from: the data to be copied
> + * @len: length of the data to be copied
> + * @off: the offset in FIFO, to which the data is copied
> + *
> + * This function copied @len bytes from the @from buffer into the @off
> + * position of the FIFO.
> + */
> +void kfifo_in_data(struct kfifo *fifo, const void *from, unsigned int len,
> +	unsigned int off)
>  {
>  	unsigned int l;
>  
> @@ -128,7 +138,7 @@ static inline void __kfifo_in_data(struc
>  
>  	smp_mb();
>  
> -	off = __kfifo_off(fifo, fifo->in + off);
> +	off = __kfifo_off(fifo, off);
>  
>  	/* first put the data starting from fifo->in to buffer end */
>  	l = min(len, fifo->size - off);
> @@ -137,6 +147,7 @@ static inline void __kfifo_in_data(struc
>  	/* then put the rest (if any) at the beginning of the buffer */
>  	memcpy(fifo->buffer, from + l, len - l);
>  }
> +EXPORT_SYMBOL_GPL(kfifo_in_data);
>  
>  static inline void __kfifo_out_data(struct kfifo *fifo,
>  		void *to, unsigned int len, unsigned int off)
> @@ -150,7 +161,7 @@ static inline void __kfifo_out_data(stru
>  
>  	smp_rmb();
>  
> -	off = __kfifo_off(fifo, fifo->out + off);
> +	off = __kfifo_off(fifo, off);
>  
>  	/* first get the data from fifo->out until the end of the buffer */
>  	l = min(len, fifo->size - off);
> @@ -232,7 +243,7 @@ unsigned int __kfifo_in_n(struct kfifo *
>  	if (kfifo_avail(fifo) < len + recsize)
>  		return len + 1;
>  
> -	__kfifo_in_data(fifo, from, len, recsize);
> +	kfifo_in_data(fifo, from, len, fifo->in + recsize);
>  	return 0;
>  }
>  EXPORT_SYMBOL(__kfifo_in_n);
> @@ -255,7 +266,7 @@ unsigned int kfifo_in(struct kfifo *fifo
>  {
>  	len = min(kfifo_avail(fifo), len);
>  
> -	__kfifo_in_data(fifo, from, len, 0);
> +	kfifo_in_data(fifo, from, len, fifo->in);
>  	__kfifo_add_in(fifo, len);
>  	return len;
>  }
> @@ -274,7 +285,7 @@ unsigned int __kfifo_out_n(struct kfifo
>  	if (kfifo_len(fifo) < len + recsize)
>  		return len;
>  
> -	__kfifo_out_data(fifo, to, len, recsize);
> +	__kfifo_out_data(fifo, to, len, fifo->out + recsize);
>  	__kfifo_add_out(fifo, len + recsize);
>  	return 0;
>  }
> @@ -296,7 +307,7 @@ unsigned int kfifo_out(struct kfifo *fif
>  {
>  	len = min(kfifo_len(fifo), len);
>  
> -	__kfifo_out_data(fifo, to, len, 0);
> +	__kfifo_out_data(fifo, to, len, fifo->out);
>  	__kfifo_add_out(fifo, len);
>  
>  	return len;
> @@ -319,7 +330,7 @@ unsigned int kfifo_out_peek(struct kfifo
>  {
>  	len = min(kfifo_len(fifo), len + offset);
>  
> -	__kfifo_out_data(fifo, to, len, offset);
> +	__kfifo_out_data(fifo, to, len, fifo->out + offset);
>  	return len;
>  }
>  EXPORT_SYMBOL(kfifo_out_peek);
> @@ -443,3 +454,205 @@ void __kfifo_skip_generic(struct kfifo *
>  }
>  EXPORT_SYMBOL(__kfifo_skip_generic);
>  
> +/**
> + * kfifo_reserve - reserves some space in the FIFO
> + * @fifo: the fifo to be used.
> + * @len: the length of space to be reserved.
> + * @ppos: return position of the reserved space.
> + *
> + * This function reserves space of @len bytes in the FIFO. The
> + * position of the reserved space is returned in @ppos. After the
> + * space is reserved, the data can be copied into reserved space with
> + * kfifo_in_data, and finally put into FIFO with kfifo_commit.
> + *
> + * This function must be paired with kfifo_commit, unless 0 is
> + * returned, which means no space is reserved.
> + *
> + * If the FIFO is used only on one CPU, kfifo_reserve/kfifo_commit
> + * pair can be used by different contexts such as NMI, IRQ, soft_irq
> + * and process (with preempt off) simultaneously safely without any
> + * locking or interrupt disabling.
> + *
> + * Preempt must be disabled between kfifo_reserve and kfifo_commit in
> + * process context for lock-less usage.
> + *
> + * Return 1 if success; return 0 if no enough space, nothing will be
> + * reserved.
> + */
> +int kfifo_reserve(struct kfifo *fifo,
> +	unsigned int len, unsigned int *ppos)
> +{
> +	unsigned int pos, npos;
> +
> +	npos = fifo->reserve;
> +	do {
> +		pos = npos;
> +		if (kfifo_size(fifo) - (pos - fifo->out) < len)
> +			return 0;
> +		npos = cmpxchg(&fifo->reserve, pos, pos + len);
> +	} while (npos != pos);
> +	*ppos = pos;
> +
> +	return 1;
> +}
> +EXPORT_SYMBOL_GPL(kfifo_reserve);
> +
> +/**
> + * kfifo_commit - commits the previous reserved space in the FIFO
> + * @fifo: the fifo to be used.
> + * @pos: position of the previous reserved space
> + *
> + * This function makes the data in previous reserved space at @pos
> + * into FIFO and can be consumed by reader.
> + */
> +void kfifo_commit(struct kfifo *fifo, unsigned int pos)
> +{
> +	unsigned int in, nin, reserve;
> +
> +	if (fifo->in == pos) {
> +		/* fifo->in must be updated after data */
> +		smp_wmb();
> +		do {
> +			in = fifo->in;
> +			/*
> +			 * fifo->in must be read before fifo->reserve,
> +			 * otherwise "in" may go beyond "reserve".
> +			 */
> +			rmb();
> +			reserve = fifo->reserve;
> +			/*
> +			 * If preempted here, fifo->reserve may go
> +			 * beyond reserve, this must be checked after
> +			 * fifo->in assignment.
> +			 */
> +			nin = cmpxchg(&fifo->in, in, reserve);
> +			/*
> +			 * If preempted here, fifo->reserve != reserve
> +			 * too, fifo->in need change with cmpxchg to
> +			 * prevent fifo->in go backwards.
> +			 */
> +		} while (reserve != fifo->reserve || in != nin);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(kfifo_commit);
> +
> +/**
> + * kfifo_ll_in - puts some data into the FIFO, lock-less version
> + * @fifo: the fifo to be used.
> + * @from: the data to be added.
> + * @len: the length of the data to be added.
> + *
> + * This function puts @len bytes from @from buffer into the FIFO. If
> + * it is used on one CPU, the function can be used simultaneously by
> + * multiple contexts such as NMI, IRQ, soft_irq, process, etc safely
> + * without any locking or interrupt disabling.
> + *
> + * Return @len if success, 0 if no enough space
> + */
> +unsigned int kfifo_ll_in(struct kfifo *fifo, const void *from, unsigned int len)
> +{
> +	unsigned int pos;
> +
> +	preempt_disable();
> +	if (!kfifo_reserve(fifo, len, &pos))
> +		return 0;
> +	kfifo_in_data(fifo, from, len, pos);
> +	kfifo_commit(fifo, pos);
> +	preempt_enable_no_resched();
> +	return len;
> +}
> +EXPORT_SYMBOL_GPL(kfifo_ll_in);
> +
> +/*
> + * Reserves some continuous spaces in the FIFO.
> + */
> +static int kfifo_reserve_continuous(struct kfifo *fifo,
> +	unsigned int *rlen, unsigned int *ppos)
> +{
> +	unsigned int pos, npos, l, to_end, avail, len = *rlen;
> +
> +	npos = fifo->reserve;
> +	do {
> +		pos = npos;
> +		avail = kfifo_size(fifo) - (pos - fifo->out);
> +		if (avail < len)
> +			return 0;
> +		to_end = kfifo_size(fifo) - __kfifo_off(fifo, pos);
> +		if (to_end < len) {
> +			if (avail - to_end < len)
> +				return 0;
> +			l = to_end;
> +		} else
> +			l = len;
> +		npos = cmpxchg(&fifo->reserve, pos, pos + l);
> +	} while (npos != pos);
> +	*ppos = pos;
> +	*rlen = l;
> +
> +	return 1;
> +}
> +
> +/**
> + * kfifo_reserve_continuous_ptr - reserves some continuous space in the FIFO
> + * @fifo: the fifo to be used.
> + * @len: the length of space to be reserved, also used to return
> + *       actual reserved length.
> + *
> + * This function reserves some continuous space of at most @len bytes
> + * in the FIFO, and return the pointer to the space. So the reserved
> + * space can be accessed "in-place", until they are committed with
> + * kfifo_commit_ptr. The resulting FIFO layout also makes it possible
> + * to be read in-place.
> + *
> + * There may be two separated free spaces in FIFO, at begin and end of
> + * the buffer. If both are not big enough, NULL will return. If the
> + * free space at end is not but the free space at begin is big enough,
> + * the free space at end will be return, with @len set to actual
> + * length. Otherwise, @len will not be changed, and free space with
> + * length @len will be returned.
> + *
> + * This function must be paired with kfifo_commit_ptr, unless NULL is
> + * returned, which means no space is reserved.
> + *
> + * If the FIFO is used only on one CPU,
> + * kfifo_reserve_continuous_ptr/kfifo_commit_ptr pair can be used by
> + * different contexts such as NMI, IRQ, soft_irq and process (with
> + * preempt off) simultaneously and safely without any locking or
> + * interrupt disabling.
> + *
> + * Preempt must be disabled between kfifo_reserve_continuous_ptr and
> + * kfifo_commit_ptr in process context for lock-less usage.
> + */
> +void *kfifo_reserve_continuous_ptr(struct kfifo *fifo,
> +	unsigned int *len)
> +{
> +	unsigned int pos;
> +
> +	if (!kfifo_reserve_continuous(fifo, len, &pos))
> +		return NULL;
> +	return __kfifo_ptr(fifo, pos);
> +}
> +EXPORT_SYMBOL_GPL(kfifo_reserve_continuous_ptr);
> +
> +/**
> + * kfifo_commit_ptr - commits the previous reserved space in the FIFO
> + * @fifo: the fifo to be used.
> + * @ptr: pointer to the previous reserved space
> + *
> + * This functions makes the previous reserved space pointed by @ptr
> + * into FIFO and can be consumed by reader.
> + */
> +void kfifo_commit_ptr(struct kfifo *fifo, void *ptr)
> +{
> +	unsigned int pos, in;
> +
> +	pos = ptr - (void *)fifo->buffer;
> +	BUG_ON(pos >= fifo->size);
> +	in = fifo->in;
> +	pos += in & ~(fifo->size - 1);
> +	if (pos < in)
> +		pos += fifo->size;
> +
> +	kfifo_commit(fifo, pos);
> +}
> +EXPORT_SYMBOL_GPL(kfifo_commit_ptr);
> 
> 


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