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trace/buffer: keep "last" entry on wraps

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When the former trace buffer implementation wrapped, the last entry
according to commit order couldn't be detected anymore. Now, the last
committed entry is always followed by an entry with length 0.

As a downside of this, there are now two meanings of "last" entry: It
means either that the entry marks the empty padding after the entry with
the highest memory address or that it actually marks the end of the
buffer according to commit order. This is an example state of the buffer
with the two types of "last" entry:

                           last                                     last
+-------------+------------+---+---------+-------------+------------+---+-------+
| len3  data3 | len4 data4 | 0 | empty   | len1  data1 | len2 data2 | 0 | empty |
+-------------+------------+---+---------+-------------+------------+---+-------+

If the entry with the highest memory address fits perfectly, the first
type of "last" entry is not needed:

                                  last
+------------+--------------------+---+-------+-------------+-------------------+
| len3 data3 | len4         data4 | 0 | empty | len1  data1 | len2        data2 |
+------------+--------------------+---+-------+-------------+-------------------+

If the buffer didn't wrap so far, there is only one "last" entry that
has both meanings:

                                                      last
+--------------------------+------------+-------------+---+---------------------+
| len1               data1 | len2 data2 | len3  data3 | 0 | empty               |
+--------------------------+------------+-------------+---+---------------------+

Issue #2735

Co-authored-by: Martin Stein <martin.stein@genode-labs.com>
This commit is contained in:
Johannes Schlatow 2018-04-10 15:44:16 +02:00 committed by Christian Helmuth
parent 373134c4e7
commit 9fc3344ee8
1 changed files with 36 additions and 1 deletions
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37
repos/base/include/base/trace/buffer.h
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@ -45,6 +45,9 @@ class Genode::Trace::Buffer
{
_head_offset = 0;
_wrapped++;
/* mark first entry with len 0 */
_head_entry()->len = 0;
}
/*
@ -96,6 +99,10 @@ class Genode::Trace::Buffer
_head_offset += sizeof(_Entry) + len;
if (_head_offset == _size)
_buffer_wrapped();
/* mark entry next to new entry with len 0 */
else if (_head_offset + sizeof(_Entry) <= _size)
_head_entry()->len = 0;
}
unsigned wrapped() const { return _wrapped; }
@ -119,7 +126,35 @@ class Genode::Trace::Buffer
size_t length() const { return _entry->len; }
char const *data() const { return _entry->data; }
bool last() const { return _entry == 0; }
/*
* XXX The meaning of this method is irritating.
*
* If it returns 'true', it means either that the entry marks
* the empty padding after the entry with the highest memory
* address or that it actually marks the end of the buffer
* according to commit order. This is an example state of the
* buffer with the two types of "last" entry:
*
* +-------------+------------+---+---------+-------------+------------+---+-------+
* | len3 data3 | len4 data4 | 0 | empty | len1 data1 | len2 data2 | 0 | empty |
* +-------------+------------+---+---------+-------------+------------+---+-------+
*
* If the entry with the highest memory address fits
* perfectly, the first type of "last" entry is not needed:
*
* +------------+--------------------+---+-------+-------------+-------------------+
* | len3 data3 | len4 data4 | 0 | empty | len1 data1 | len2 data2 |
* +------------+--------------------+---+-------+-------------+-------------------+
*
* If the buffer didn't wrap so far, there is only one "last"
* entry that has both meanings:
*
* +--------------------------+------------+-------------+---+---------------------+
* | len1 data1 | len2 data2 | len3 data3 | 0 | empty |
* +--------------------------+------------+-------------+---+---------------------+
*/
bool last() const { return _entry == 0; }
/*
* \deprecated use 'last' instead