src/svm/svm_fifo.c - fdio/vpp - Gitiles

 /*
  * Copyright (c) 2016 Cisco and/or its affiliates.
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at:
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include <svm/svm_fifo.h>

 /** create an svm fifo, in the current heap. Fails vs blow up the process */
 svm_fifo_t *
 svm_fifo_create (u32 data_size_in_bytes)
 {
   svm_fifo_t *f;
   pthread_mutexattr_t attr;
   pthread_condattr_t cattr;

   f = clib_mem_alloc_aligned_or_null (sizeof (*f) + data_size_in_bytes,
 				      CLIB_CACHE_LINE_BYTES);
   if (f == 0)
     return 0;

   memset (f, 0, sizeof (*f) + data_size_in_bytes);
   f->nitems = data_size_in_bytes;
   f->ooos_list_head = OOO_SEGMENT_INVALID_INDEX;

   memset (&attr, 0, sizeof (attr));
   memset (&cattr, 0, sizeof (cattr));

   if (pthread_mutexattr_init (&attr))
     clib_unix_warning ("mutexattr_init");
   if (pthread_mutexattr_setpshared (&attr, PTHREAD_PROCESS_SHARED))
     clib_unix_warning ("pthread_mutexattr_setpshared");
   if (pthread_mutex_init (&f->mutex, &attr))
     clib_unix_warning ("mutex_init");
   if (pthread_mutexattr_destroy (&attr))
     clib_unix_warning ("mutexattr_destroy");
   if (pthread_condattr_init (&cattr))
     clib_unix_warning ("condattr_init");
   if (pthread_condattr_setpshared (&cattr, PTHREAD_PROCESS_SHARED))
     clib_unix_warning ("condattr_setpshared");
   if (pthread_cond_init (&f->condvar, &cattr))
     clib_unix_warning ("cond_init1");
   if (pthread_condattr_destroy (&cattr))
     clib_unix_warning ("cond_init2");

   return (f);
 }

 always_inline ooo_segment_t *
 ooo_segment_new (svm_fifo_t * f, u32 start, u32 length)
 {
   ooo_segment_t *s;

   pool_get (f->ooo_segments, s);

   s->fifo_position = start;
   s->length = length;

   s->prev = s->next = OOO_SEGMENT_INVALID_INDEX;

   return s;
 }

 always_inline void
 ooo_segment_del (svm_fifo_t * f, u32 index)
 {
   ooo_segment_t *cur, *prev = 0, *next = 0;
   cur = pool_elt_at_index (f->ooo_segments, index);

   if (cur->next != OOO_SEGMENT_INVALID_INDEX)
     {
       next = pool_elt_at_index (f->ooo_segments, cur->next);
       next->prev = cur->prev;
     }

   if (cur->prev != OOO_SEGMENT_INVALID_INDEX)
     {
       prev = pool_elt_at_index (f->ooo_segments, cur->prev);
       prev->next = cur->next;
     }
   else
     {
       f->ooos_list_head = cur->next;
     }

   pool_put (f->ooo_segments, cur);
 }

 /**
  * Add segment to fifo's out-of-order segment list. Takes care of merging
  * adjacent segments and removing overlapping ones.
  */
 static void
 ooo_segment_add (svm_fifo_t * f, u32 offset, u32 length)
 {
   ooo_segment_t *s, *new_s, *prev, *next, *it;
   u32 new_index, position, end_offset, s_sof, s_eof, s_index;

   position = (f->tail + offset) % f->nitems;
   end_offset = offset + length;

   if (f->ooos_list_head == OOO_SEGMENT_INVALID_INDEX)
     {
       s = ooo_segment_new (f, position, length);
       f->ooos_list_head = s - f->ooo_segments;
       f->ooos_newest = f->ooos_list_head;
       return;
     }

   /* Find first segment that starts after new segment */
   s = pool_elt_at_index (f->ooo_segments, f->ooos_list_head);
   while (s->next != OOO_SEGMENT_INVALID_INDEX
 	 && ooo_segment_offset (f, s) <= offset)
     s = pool_elt_at_index (f->ooo_segments, s->next);

   s_index = s - f->ooo_segments;
   s_sof = ooo_segment_offset (f, s);
   s_eof = ooo_segment_end_offset (f, s);

   /* No overlap, add before current segment */
   if (end_offset < s_sof)
     {
       new_s = ooo_segment_new (f, position, length);
       new_index = new_s - f->ooo_segments;

       /* Pool might've moved, get segment again */
       s = pool_elt_at_index (f->ooo_segments, s_index);

       if (s->prev != OOO_SEGMENT_INVALID_INDEX)
 	{
 	  new_s->prev = s->prev;

 	  prev = pool_elt_at_index (f->ooo_segments, new_s->prev);
 	  prev->next = new_index;
 	}
       else
 	{
 	  /* New head */
 	  f->ooos_list_head = new_index;
 	}

       new_s->next = s - f->ooo_segments;
       s->prev = new_index;
       f->ooos_newest = new_index;
       return;
     }
   /* No overlap, add after current segment */
   else if (s_eof < offset)
     {
       new_s = ooo_segment_new (f, position, length);
       new_index = new_s - f->ooo_segments;

       /* Pool might've moved, get segment again */
       s = pool_elt_at_index (f->ooo_segments, s_index);

       if (s->next != OOO_SEGMENT_INVALID_INDEX)
 	{
 	  new_s->next = s->next;

 	  next = pool_elt_at_index (f->ooo_segments, new_s->next);
 	  next->prev = new_index;
 	}

       new_s->prev = s - f->ooo_segments;
       s->next = new_index;
       f->ooos_newest = new_index;

       return;
     }

   /*
    * Merge needed
    */

   /* Merge at head */
   if (offset <= s_sof)
     {
       /* If we have a previous, check if we overlap */
       if (s->prev != OOO_SEGMENT_INVALID_INDEX)
 	{
 	  prev = pool_elt_at_index (f->ooo_segments, s->prev);

 	  /* New segment merges prev and current. Remove previous and
 	   * update position of current. */
 	  if (ooo_segment_end_offset (f, prev) >= offset)
 	    {
 	      s->fifo_position = prev->fifo_position;
 	      s->length = s_eof - ooo_segment_offset (f, prev);
 	      ooo_segment_del (f, s->prev);
 	    }
 	}
       else
 	{
 	  s->fifo_position = position;
 	  s->length = s_eof - ooo_segment_offset (f, s);
 	}

       /* The new segment's tail may cover multiple smaller ones */
       if (s_eof < end_offset)
 	{
 	  /* Remove segments completely covered */
 	  it = (s->next != OOO_SEGMENT_INVALID_INDEX) ?
 	    pool_elt_at_index (f->ooo_segments, s->next) : 0;
 	  while (it && ooo_segment_end_offset (f, it) < end_offset)
 	    {
 	      next = (it->next != OOO_SEGMENT_INVALID_INDEX) ?
 		pool_elt_at_index (f->ooo_segments, it->next) : 0;
 	      ooo_segment_del (f, it - f->ooo_segments);
 	      it = next;
 	    }

 	  /* Update length. Segment's start might have changed. */
 	  s->length = end_offset - ooo_segment_offset (f, s);

 	  /* If partial overlap with last, merge */
 	  if (it && ooo_segment_offset (f, it) < end_offset)
 	    {
 	      s->length +=
 		it->length - (ooo_segment_offset (f, it) - end_offset);
 	      ooo_segment_del (f, it - f->ooo_segments);
 	    }
 	}
     }
   /* Last but overlapping previous */
   else if (s_eof <= end_offset)
     {
       s->length = end_offset - ooo_segment_offset (f, s);
     }
   /* New segment completely covered by current one */
   else
     {
       /* Do Nothing */
     }

   /* Most recently updated segment */
   f->ooos_newest = s - f->ooo_segments;
 }

 /**
  * Removes segments that can now be enqueued because the fifo's tail has
  * advanced. Returns the number of bytes added to tail.
  */
 static int
 ooo_segment_try_collect (svm_fifo_t * f, u32 n_bytes_enqueued)
 {
   ooo_segment_t *s;
   u32 index, bytes = 0, diff;
   u32 cursize;

   /* read cursize, which can only increase while we're working */
   cursize = svm_fifo_max_dequeue (f);

   s = pool_elt_at_index (f->ooo_segments, f->ooos_list_head);

   /* If last tail update overlaps one/multiple ooo segments, remove them */
   diff = (f->nitems + f->tail - s->fifo_position) % f->nitems;
   while (0 < diff && diff < n_bytes_enqueued)
     {
       /* Segment end is beyond the tail. Advance tail and be done */
       if (diff < s->length)
 	{
 	  f->tail += s->length - diff;
 	  f->tail %= f->nitems;
 	  break;
 	}
       /* If we have next go on */
       else if (s->next != OOO_SEGMENT_INVALID_INDEX)
 	{
 	  index = s - f->ooo_segments;
 	  s = pool_elt_at_index (f->ooo_segments, s->next);
 	  diff = (f->nitems + f->tail - s->fifo_position) % f->nitems;
 	  ooo_segment_del (f, index);
 	}
       /* End of search */
       else
 	{
 	  break;
 	}
     }

   /* If tail is adjacent to an ooo segment, 'consume' it */
   if (diff == 0)
     {
       bytes = ((f->nitems - cursize) >= s->length) ? s->length :
 	f->nitems - cursize;

       f->tail += bytes;
       f->tail %= f->nitems;

       ooo_segment_del (f, s - f->ooo_segments);
     }

   return bytes;
 }

 static int
 svm_fifo_enqueue_internal (svm_fifo_t * f,
 			   int pid, u32 max_bytes, u8 * copy_from_here)
 {
   u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
   u32 cursize, nitems;

   /* read cursize, which can only increase while we're working */
   cursize = svm_fifo_max_dequeue (f);

   if (PREDICT_FALSE (cursize == f->nitems))
     return -2;			/* fifo stuffed */

   nitems = f->nitems;

   /* Number of bytes we're going to copy */
   total_copy_bytes = (nitems - cursize) < max_bytes ?
     (nitems - cursize) : max_bytes;

   if (PREDICT_TRUE (copy_from_here != 0))
     {
       /* Number of bytes in first copy segment */
       first_copy_bytes = ((nitems - f->tail) < total_copy_bytes)
 	? (nitems - f->tail) : total_copy_bytes;

       clib_memcpy (&f->data[f->tail], copy_from_here, first_copy_bytes);
       f->tail += first_copy_bytes;
       f->tail = (f->tail == nitems) ? 0 : f->tail;

       /* Number of bytes in second copy segment, if any */
       second_copy_bytes = total_copy_bytes - first_copy_bytes;
       if (second_copy_bytes)
 	{
 	  clib_memcpy (&f->data[f->tail], copy_from_here + first_copy_bytes,
 		       second_copy_bytes);
 	  f->tail += second_copy_bytes;
 	  f->tail = (f->tail == nitems) ? 0 : f->tail;
 	}
     }
   else
     {
       /* Account for a zero-copy enqueue done elsewhere */
       ASSERT (max_bytes <= (nitems - cursize));
       f->tail += max_bytes;
       f->tail = f->tail % nitems;
       total_copy_bytes = max_bytes;
     }

   /* Any out-of-order segments to collect? */
   if (PREDICT_FALSE (f->ooos_list_head != OOO_SEGMENT_INVALID_INDEX))
     total_copy_bytes += ooo_segment_try_collect (f, total_copy_bytes);

   /* Atomically increase the queue length */
   __sync_fetch_and_add (&f->cursize, total_copy_bytes);

   return (total_copy_bytes);
 }

 int
 svm_fifo_enqueue_nowait (svm_fifo_t * f,
 			 int pid, u32 max_bytes, u8 * copy_from_here)
 {
   return svm_fifo_enqueue_internal (f, pid, max_bytes, copy_from_here);
 }

 /**
  * Enqueue a future segment.
  *
  * Two choices: either copies the entire segment, or copies nothing
  * Returns 0 of the entire segment was copied
  * Returns -1 if none of the segment was copied due to lack of space
  */
 static int
 svm_fifo_enqueue_with_offset_internal (svm_fifo_t * f,
 				       int pid,
 				       u32 offset,
 				       u32 required_bytes,
 				       u8 * copy_from_here)
 {
   u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
   u32 cursize, nitems;
   u32 tail_plus_offset;

   ASSERT (offset > 0);

   /* read cursize, which can only increase while we're working */
   cursize = svm_fifo_max_dequeue (f);
   nitems = f->nitems;

   /* Will this request fit? */
   if ((required_bytes + offset) > (nitems - cursize))
     return -1;

   ooo_segment_add (f, offset, required_bytes);

   /* Number of bytes we're going to copy */
   total_copy_bytes = required_bytes;
   tail_plus_offset = (f->tail + offset) % nitems;

   /* Number of bytes in first copy segment */
   first_copy_bytes = ((nitems - tail_plus_offset) < total_copy_bytes)
     ? (nitems - tail_plus_offset) : total_copy_bytes;

   clib_memcpy (&f->data[tail_plus_offset], copy_from_here, first_copy_bytes);

   /* Number of bytes in second copy segment, if any */
   second_copy_bytes = total_copy_bytes - first_copy_bytes;
   if (second_copy_bytes)
     {
       tail_plus_offset += first_copy_bytes;
       tail_plus_offset %= nitems;

       ASSERT (tail_plus_offset == 0);

       clib_memcpy (&f->data[tail_plus_offset],
 		   copy_from_here + first_copy_bytes, second_copy_bytes);
     }

   return (0);
 }


 int
 svm_fifo_enqueue_with_offset (svm_fifo_t * f,
 			      int pid,
 			      u32 offset,
 			      u32 required_bytes, u8 * copy_from_here)
 {
   return svm_fifo_enqueue_with_offset_internal
     (f, pid, offset, required_bytes, copy_from_here);
 }


 static int
 svm_fifo_dequeue_internal (svm_fifo_t * f,
 			   int pid, u32 max_bytes, u8 * copy_here)
 {
   u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
   u32 cursize, nitems;

   /* read cursize, which can only increase while we're working */
   cursize = svm_fifo_max_dequeue (f);
   if (PREDICT_FALSE (cursize == 0))
     return -2;			/* nothing in the fifo */

   nitems = f->nitems;

   /* Number of bytes we're going to copy */
   total_copy_bytes = (cursize < max_bytes) ? cursize : max_bytes;

   if (PREDICT_TRUE (copy_here != 0))
     {
       /* Number of bytes in first copy segment */
       first_copy_bytes = ((nitems - f->head) < total_copy_bytes)
 	? (nitems - f->head) : total_copy_bytes;
       clib_memcpy (copy_here, &f->data[f->head], first_copy_bytes);
       f->head += first_copy_bytes;
       f->head = (f->head == nitems) ? 0 : f->head;

       /* Number of bytes in second copy segment, if any */
       second_copy_bytes = total_copy_bytes - first_copy_bytes;
       if (second_copy_bytes)
 	{
 	  clib_memcpy (copy_here + first_copy_bytes,
 		       &f->data[f->head], second_copy_bytes);
 	  f->head += second_copy_bytes;
 	  f->head = (f->head == nitems) ? 0 : f->head;
 	}
     }
   else
     {
       /* Account for a zero-copy dequeue done elsewhere */
       ASSERT (max_bytes <= cursize);
       f->head += max_bytes;
       f->head = f->head % nitems;
       cursize -= max_bytes;
       total_copy_bytes = max_bytes;
     }

   __sync_fetch_and_sub (&f->cursize, total_copy_bytes);

   return (total_copy_bytes);
 }

 int
 svm_fifo_dequeue_nowait (svm_fifo_t * f,
 			 int pid, u32 max_bytes, u8 * copy_here)
 {
   return svm_fifo_dequeue_internal (f, pid, max_bytes, copy_here);
 }

 int
 svm_fifo_peek (svm_fifo_t * f, int pid, u32 offset, u32 max_bytes,
 	       u8 * copy_here)
 {
   u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
   u32 cursize, nitems, real_head;

   /* read cursize, which can only increase while we're working */
   cursize = svm_fifo_max_dequeue (f);
   if (PREDICT_FALSE (cursize == 0))
     return -2;			/* nothing in the fifo */

   nitems = f->nitems;
   real_head = f->head + offset;
   real_head = real_head >= nitems ? real_head - nitems : real_head;

   /* Number of bytes we're going to copy */
   total_copy_bytes = (cursize < max_bytes) ? cursize : max_bytes;

   if (PREDICT_TRUE (copy_here != 0))
     {
       /* Number of bytes in first copy segment */
       first_copy_bytes =
 	((nitems - real_head) < total_copy_bytes) ?
 	(nitems - real_head) : total_copy_bytes;
       clib_memcpy (copy_here, &f->data[real_head], first_copy_bytes);

       /* Number of bytes in second copy segment, if any */
       second_copy_bytes = total_copy_bytes - first_copy_bytes;
       if (second_copy_bytes)
 	{
 	  clib_memcpy (copy_here + first_copy_bytes, &f->data[0],
 		       second_copy_bytes);
 	}
     }
   return total_copy_bytes;
 }

 int
 svm_fifo_dequeue_drop (svm_fifo_t * f, int pid, u32 max_bytes)
 {
   u32 total_drop_bytes, first_drop_bytes, second_drop_bytes;
   u32 cursize, nitems;

   /* read cursize, which can only increase while we're working */
   cursize = svm_fifo_max_dequeue (f);
   if (PREDICT_FALSE (cursize == 0))
     return -2;			/* nothing in the fifo */

   nitems = f->nitems;

   /* Number of bytes we're going to drop */
   total_drop_bytes = (cursize < max_bytes) ? cursize : max_bytes;

   /* Number of bytes in first copy segment */
   first_drop_bytes =
     ((nitems - f->head) < total_drop_bytes) ?
     (nitems - f->head) : total_drop_bytes;
   f->head += first_drop_bytes;
   f->head = (f->head == nitems) ? 0 : f->head;

   /* Number of bytes in second drop segment, if any */
   second_drop_bytes = total_drop_bytes - first_drop_bytes;
   if (second_drop_bytes)
     {
       f->head += second_drop_bytes;
       f->head = (f->head == nitems) ? 0 : f->head;
     }

   __sync_fetch_and_sub (&f->cursize, total_drop_bytes);

   return total_drop_bytes;
 }

 /*
  * fd.io coding-style-patch-verification: ON
  *
  * Local Variables:
  * eval: (c-set-style "gnu")
  * End:
  */
	/*
	* Copyright (c) 2016 Cisco and/or its affiliates.
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at:
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include <svm/svm_fifo.h>

	/** create an svm fifo, in the current heap. Fails vs blow up the process */
	svm_fifo_t *
	svm_fifo_create (u32 data_size_in_bytes)
	{
	svm_fifo_t *f;
	pthread_mutexattr_t attr;
	pthread_condattr_t cattr;

	f = clib_mem_alloc_aligned_or_null (sizeof (*f) + data_size_in_bytes,
	CLIB_CACHE_LINE_BYTES);
	if (f == 0)
	return 0;

	memset (f, 0, sizeof (*f) + data_size_in_bytes);
	f->nitems = data_size_in_bytes;
	f->ooos_list_head = OOO_SEGMENT_INVALID_INDEX;

	memset (&attr, 0, sizeof (attr));
	memset (&cattr, 0, sizeof (cattr));

	if (pthread_mutexattr_init (&attr))
	clib_unix_warning ("mutexattr_init");
	if (pthread_mutexattr_setpshared (&attr, PTHREAD_PROCESS_SHARED))
	clib_unix_warning ("pthread_mutexattr_setpshared");
	if (pthread_mutex_init (&f->mutex, &attr))
	clib_unix_warning ("mutex_init");
	if (pthread_mutexattr_destroy (&attr))
	clib_unix_warning ("mutexattr_destroy");
	if (pthread_condattr_init (&cattr))
	clib_unix_warning ("condattr_init");
	if (pthread_condattr_setpshared (&cattr, PTHREAD_PROCESS_SHARED))
	clib_unix_warning ("condattr_setpshared");
	if (pthread_cond_init (&f->condvar, &cattr))
	clib_unix_warning ("cond_init1");
	if (pthread_condattr_destroy (&cattr))
	clib_unix_warning ("cond_init2");

	return (f);
	}

	always_inline ooo_segment_t *
	ooo_segment_new (svm_fifo_t * f, u32 start, u32 length)
	{
	ooo_segment_t *s;

	pool_get (f->ooo_segments, s);

	s->fifo_position = start;
	s->length = length;

	s->prev = s->next = OOO_SEGMENT_INVALID_INDEX;

	return s;
	}

	always_inline void
	ooo_segment_del (svm_fifo_t * f, u32 index)
	{
	ooo_segment_t cur, prev = 0, *next = 0;
	cur = pool_elt_at_index (f->ooo_segments, index);

	if (cur->next != OOO_SEGMENT_INVALID_INDEX)
	{
	next = pool_elt_at_index (f->ooo_segments, cur->next);
	next->prev = cur->prev;
	}

	if (cur->prev != OOO_SEGMENT_INVALID_INDEX)
	{
	prev = pool_elt_at_index (f->ooo_segments, cur->prev);
	prev->next = cur->next;
	}
	else
	{
	f->ooos_list_head = cur->next;
	}

	pool_put (f->ooo_segments, cur);
	}

	/**
	* Add segment to fifo's out-of-order segment list. Takes care of merging
	* adjacent segments and removing overlapping ones.
	*/
	static void
	ooo_segment_add (svm_fifo_t * f, u32 offset, u32 length)
	{
	ooo_segment_t s, new_s, prev, next, *it;
	u32 new_index, position, end_offset, s_sof, s_eof, s_index;

	position = (f->tail + offset) % f->nitems;
	end_offset = offset + length;

	if (f->ooos_list_head == OOO_SEGMENT_INVALID_INDEX)
	{
	s = ooo_segment_new (f, position, length);
	f->ooos_list_head = s - f->ooo_segments;
	f->ooos_newest = f->ooos_list_head;
	return;
	}

	/* Find first segment that starts after new segment */
	s = pool_elt_at_index (f->ooo_segments, f->ooos_list_head);
	while (s->next != OOO_SEGMENT_INVALID_INDEX
	&& ooo_segment_offset (f, s) <= offset)
	s = pool_elt_at_index (f->ooo_segments, s->next);

	s_index = s - f->ooo_segments;
	s_sof = ooo_segment_offset (f, s);
	s_eof = ooo_segment_end_offset (f, s);

	/* No overlap, add before current segment */
	if (end_offset < s_sof)
	{
	new_s = ooo_segment_new (f, position, length);
	new_index = new_s - f->ooo_segments;

	/* Pool might've moved, get segment again */
	s = pool_elt_at_index (f->ooo_segments, s_index);

	if (s->prev != OOO_SEGMENT_INVALID_INDEX)
	{
	new_s->prev = s->prev;

	prev = pool_elt_at_index (f->ooo_segments, new_s->prev);
	prev->next = new_index;
	}
	else
	{
	/* New head */
	f->ooos_list_head = new_index;
	}

	new_s->next = s - f->ooo_segments;
	s->prev = new_index;
	f->ooos_newest = new_index;
	return;
	}
	/* No overlap, add after current segment */
	else if (s_eof < offset)
	{
	new_s = ooo_segment_new (f, position, length);
	new_index = new_s - f->ooo_segments;

	/* Pool might've moved, get segment again */
	s = pool_elt_at_index (f->ooo_segments, s_index);

	if (s->next != OOO_SEGMENT_INVALID_INDEX)
	{
	new_s->next = s->next;

	next = pool_elt_at_index (f->ooo_segments, new_s->next);
	next->prev = new_index;
	}

	new_s->prev = s - f->ooo_segments;
	s->next = new_index;
	f->ooos_newest = new_index;

	return;
	}

	/*
	* Merge needed
	*/

	/* Merge at head */
	if (offset <= s_sof)
	{
	/* If we have a previous, check if we overlap */
	if (s->prev != OOO_SEGMENT_INVALID_INDEX)
	{
	prev = pool_elt_at_index (f->ooo_segments, s->prev);

	/* New segment merges prev and current. Remove previous and
	* update position of current. */
	if (ooo_segment_end_offset (f, prev) >= offset)
	{
	s->fifo_position = prev->fifo_position;
	s->length = s_eof - ooo_segment_offset (f, prev);
	ooo_segment_del (f, s->prev);
	}
	}
	else
	{
	s->fifo_position = position;
	s->length = s_eof - ooo_segment_offset (f, s);
	}

	/* The new segment's tail may cover multiple smaller ones */
	if (s_eof < end_offset)
	{
	/* Remove segments completely covered */
	it = (s->next != OOO_SEGMENT_INVALID_INDEX) ?
	pool_elt_at_index (f->ooo_segments, s->next) : 0;
	while (it && ooo_segment_end_offset (f, it) < end_offset)
	{
	next = (it->next != OOO_SEGMENT_INVALID_INDEX) ?
	pool_elt_at_index (f->ooo_segments, it->next) : 0;
	ooo_segment_del (f, it - f->ooo_segments);
	it = next;
	}

	/* Update length. Segment's start might have changed. */
	s->length = end_offset - ooo_segment_offset (f, s);

	/* If partial overlap with last, merge */
	if (it && ooo_segment_offset (f, it) < end_offset)
	{
	s->length +=
	it->length - (ooo_segment_offset (f, it) - end_offset);
	ooo_segment_del (f, it - f->ooo_segments);
	}
	}
	}
	/* Last but overlapping previous */
	else if (s_eof <= end_offset)
	{
	s->length = end_offset - ooo_segment_offset (f, s);
	}
	/* New segment completely covered by current one */
	else
	{
	/* Do Nothing */
	}

	/* Most recently updated segment */
	f->ooos_newest = s - f->ooo_segments;
	}

	/**
	* Removes segments that can now be enqueued because the fifo's tail has
	* advanced. Returns the number of bytes added to tail.
	*/
	static int
	ooo_segment_try_collect (svm_fifo_t * f, u32 n_bytes_enqueued)
	{
	ooo_segment_t *s;
	u32 index, bytes = 0, diff;
	u32 cursize;

	/* read cursize, which can only increase while we're working */
	cursize = svm_fifo_max_dequeue (f);

	s = pool_elt_at_index (f->ooo_segments, f->ooos_list_head);

	/* If last tail update overlaps one/multiple ooo segments, remove them */
	diff = (f->nitems + f->tail - s->fifo_position) % f->nitems;
	while (0 < diff && diff < n_bytes_enqueued)
	{
	/* Segment end is beyond the tail. Advance tail and be done */
	if (diff < s->length)
	{
	f->tail += s->length - diff;
	f->tail %= f->nitems;
	break;
	}
	/* If we have next go on */
	else if (s->next != OOO_SEGMENT_INVALID_INDEX)
	{
	index = s - f->ooo_segments;
	s = pool_elt_at_index (f->ooo_segments, s->next);
	diff = (f->nitems + f->tail - s->fifo_position) % f->nitems;
	ooo_segment_del (f, index);
	}
	/* End of search */
	else
	{
	break;
	}
	}

	/* If tail is adjacent to an ooo segment, 'consume' it */
	if (diff == 0)
	{
	bytes = ((f->nitems - cursize) >= s->length) ? s->length :
	f->nitems - cursize;

	f->tail += bytes;
	f->tail %= f->nitems;

	ooo_segment_del (f, s - f->ooo_segments);
	}

	return bytes;
	}

	static int
	svm_fifo_enqueue_internal (svm_fifo_t * f,
	int pid, u32 max_bytes, u8 * copy_from_here)
	{
	u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
	u32 cursize, nitems;

	/* read cursize, which can only increase while we're working */
	cursize = svm_fifo_max_dequeue (f);

	if (PREDICT_FALSE (cursize == f->nitems))
	return -2; /* fifo stuffed */

	nitems = f->nitems;

	/* Number of bytes we're going to copy */
	total_copy_bytes = (nitems - cursize) < max_bytes ?
	(nitems - cursize) : max_bytes;

	if (PREDICT_TRUE (copy_from_here != 0))
	{
	/* Number of bytes in first copy segment */
	first_copy_bytes = ((nitems - f->tail) < total_copy_bytes)
	? (nitems - f->tail) : total_copy_bytes;

	clib_memcpy (&f->data[f->tail], copy_from_here, first_copy_bytes);
	f->tail += first_copy_bytes;
	f->tail = (f->tail == nitems) ? 0 : f->tail;

	/* Number of bytes in second copy segment, if any */
	second_copy_bytes = total_copy_bytes - first_copy_bytes;
	if (second_copy_bytes)
	{
	clib_memcpy (&f->data[f->tail], copy_from_here + first_copy_bytes,
	second_copy_bytes);
	f->tail += second_copy_bytes;
	f->tail = (f->tail == nitems) ? 0 : f->tail;
	}
	}
	else
	{
	/* Account for a zero-copy enqueue done elsewhere */
	ASSERT (max_bytes <= (nitems - cursize));
	f->tail += max_bytes;
	f->tail = f->tail % nitems;
	total_copy_bytes = max_bytes;
	}

	/* Any out-of-order segments to collect? */
	if (PREDICT_FALSE (f->ooos_list_head != OOO_SEGMENT_INVALID_INDEX))
	total_copy_bytes += ooo_segment_try_collect (f, total_copy_bytes);

	/* Atomically increase the queue length */
	__sync_fetch_and_add (&f->cursize, total_copy_bytes);

	return (total_copy_bytes);
	}

	int
	svm_fifo_enqueue_nowait (svm_fifo_t * f,
	int pid, u32 max_bytes, u8 * copy_from_here)
	{
	return svm_fifo_enqueue_internal (f, pid, max_bytes, copy_from_here);
	}

	/**
	* Enqueue a future segment.
	*
	* Two choices: either copies the entire segment, or copies nothing
	* Returns 0 of the entire segment was copied
	* Returns -1 if none of the segment was copied due to lack of space
	*/
	static int
	svm_fifo_enqueue_with_offset_internal (svm_fifo_t * f,
	int pid,
	u32 offset,
	u32 required_bytes,
	u8 * copy_from_here)
	{
	u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
	u32 cursize, nitems;
	u32 tail_plus_offset;

	ASSERT (offset > 0);

	/* read cursize, which can only increase while we're working */
	cursize = svm_fifo_max_dequeue (f);
	nitems = f->nitems;

	/* Will this request fit? */
	if ((required_bytes + offset) > (nitems - cursize))
	return -1;

	ooo_segment_add (f, offset, required_bytes);

	/* Number of bytes we're going to copy */
	total_copy_bytes = required_bytes;
	tail_plus_offset = (f->tail + offset) % nitems;

	/* Number of bytes in first copy segment */
	first_copy_bytes = ((nitems - tail_plus_offset) < total_copy_bytes)
	? (nitems - tail_plus_offset) : total_copy_bytes;

	clib_memcpy (&f->data[tail_plus_offset], copy_from_here, first_copy_bytes);

	/* Number of bytes in second copy segment, if any */
	second_copy_bytes = total_copy_bytes - first_copy_bytes;
	if (second_copy_bytes)
	{
	tail_plus_offset += first_copy_bytes;
	tail_plus_offset %= nitems;

	ASSERT (tail_plus_offset == 0);

	clib_memcpy (&f->data[tail_plus_offset],
	copy_from_here + first_copy_bytes, second_copy_bytes);
	}

	return (0);
	}


	int
	svm_fifo_enqueue_with_offset (svm_fifo_t * f,
	int pid,
	u32 offset,
	u32 required_bytes, u8 * copy_from_here)
	{
	return svm_fifo_enqueue_with_offset_internal
	(f, pid, offset, required_bytes, copy_from_here);
	}


	static int
	svm_fifo_dequeue_internal (svm_fifo_t * f,
	int pid, u32 max_bytes, u8 * copy_here)
	{
	u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
	u32 cursize, nitems;

	/* read cursize, which can only increase while we're working */
	cursize = svm_fifo_max_dequeue (f);
	if (PREDICT_FALSE (cursize == 0))
	return -2; /* nothing in the fifo */

	nitems = f->nitems;

	/* Number of bytes we're going to copy */
	total_copy_bytes = (cursize < max_bytes) ? cursize : max_bytes;

	if (PREDICT_TRUE (copy_here != 0))
	{
	/* Number of bytes in first copy segment */
	first_copy_bytes = ((nitems - f->head) < total_copy_bytes)
	? (nitems - f->head) : total_copy_bytes;
	clib_memcpy (copy_here, &f->data[f->head], first_copy_bytes);
	f->head += first_copy_bytes;
	f->head = (f->head == nitems) ? 0 : f->head;

	/* Number of bytes in second copy segment, if any */
	second_copy_bytes = total_copy_bytes - first_copy_bytes;
	if (second_copy_bytes)
	{
	clib_memcpy (copy_here + first_copy_bytes,
	&f->data[f->head], second_copy_bytes);
	f->head += second_copy_bytes;
	f->head = (f->head == nitems) ? 0 : f->head;
	}
	}
	else
	{
	/* Account for a zero-copy dequeue done elsewhere */
	ASSERT (max_bytes <= cursize);
	f->head += max_bytes;
	f->head = f->head % nitems;
	cursize -= max_bytes;
	total_copy_bytes = max_bytes;
	}

	__sync_fetch_and_sub (&f->cursize, total_copy_bytes);

	return (total_copy_bytes);
	}

	int
	svm_fifo_dequeue_nowait (svm_fifo_t * f,
	int pid, u32 max_bytes, u8 * copy_here)
	{
	return svm_fifo_dequeue_internal (f, pid, max_bytes, copy_here);
	}

	int
	svm_fifo_peek (svm_fifo_t * f, int pid, u32 offset, u32 max_bytes,
	u8 * copy_here)
	{
	u32 total_copy_bytes, first_copy_bytes, second_copy_bytes;
	u32 cursize, nitems, real_head;

	/* read cursize, which can only increase while we're working */
	cursize = svm_fifo_max_dequeue (f);
	if (PREDICT_FALSE (cursize == 0))
	return -2; /* nothing in the fifo */

	nitems = f->nitems;
	real_head = f->head + offset;
	real_head = real_head >= nitems ? real_head - nitems : real_head;

	/* Number of bytes we're going to copy */
	total_copy_bytes = (cursize < max_bytes) ? cursize : max_bytes;

	if (PREDICT_TRUE (copy_here != 0))
	{
	/* Number of bytes in first copy segment */
	first_copy_bytes =
	((nitems - real_head) < total_copy_bytes) ?
	(nitems - real_head) : total_copy_bytes;
	clib_memcpy (copy_here, &f->data[real_head], first_copy_bytes);

	/* Number of bytes in second copy segment, if any */
	second_copy_bytes = total_copy_bytes - first_copy_bytes;
	if (second_copy_bytes)
	{
	clib_memcpy (copy_here + first_copy_bytes, &f->data[0],
	second_copy_bytes);
	}
	}
	return total_copy_bytes;
	}

	int
	svm_fifo_dequeue_drop (svm_fifo_t * f, int pid, u32 max_bytes)
	{
	u32 total_drop_bytes, first_drop_bytes, second_drop_bytes;
	u32 cursize, nitems;

	/* read cursize, which can only increase while we're working */
	cursize = svm_fifo_max_dequeue (f);
	if (PREDICT_FALSE (cursize == 0))
	return -2; /* nothing in the fifo */

	nitems = f->nitems;

	/* Number of bytes we're going to drop */
	total_drop_bytes = (cursize < max_bytes) ? cursize : max_bytes;

	/* Number of bytes in first copy segment */
	first_drop_bytes =
	((nitems - f->head) < total_drop_bytes) ?
	(nitems - f->head) : total_drop_bytes;
	f->head += first_drop_bytes;
	f->head = (f->head == nitems) ? 0 : f->head;

	/* Number of bytes in second drop segment, if any */
	second_drop_bytes = total_drop_bytes - first_drop_bytes;
	if (second_drop_bytes)
	{
	f->head += second_drop_bytes;
	f->head = (f->head == nitems) ? 0 : f->head;
	}

	__sync_fetch_and_sub (&f->cursize, total_drop_bytes);

	return total_drop_bytes;
	}

	/*
	* fd.io coding-style-patch-verification: ON
	*
	* Local Variables:
	* eval: (c-set-style "gnu")
	* End:
	*/