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scheduler instrumentation: Add a little more protection for the SMP case

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This commit is contained in:
Gregory Nutt 2016-11-30 07:24:15 -06:00
parent b29b77532f
commit 8453343426
1 changed files with 34 additions and 6 deletions
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40
sched/sched/sched_note.c
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@ -63,8 +63,8 @@
struct note_info_s
{
unsigned int ni_head;
unsigned int ni_tail;
volatile unsigned int ni_head;
volatile unsigned int ni_tail;
uint8_t ni_buffer[CONFIG_SCHED_NOTE_BUFSIZE];
};
@ -281,6 +281,7 @@ static void note_add(FAR const uint8_t *note, uint8_t notelen)
{
unsigned int head;
unsigned int next;
unsigned int nxthd;
#ifdef CONFIG_SMP
/* Ignore notes that are not in the set of monitored CPUs */
@ -293,10 +294,37 @@ static void note_add(FAR const uint8_t *note, uint8_t notelen)
}
#endif
/* Get the index to the head of the circular buffer */
DEBUGASSERT(note != NULL && notelen < CONFIG_SCHED_NOTE_BUFSIZE);
head = g_note_info.ni_head;
do
{
/* Get the index to the head of the circular buffer */
head = g_note_info.ni_head;
/* Pre-calculate the next head index. We do this in advance to
* protect note as it is written. In the single CPU case, this is not
* a problem because this logic is always called within a critical
* section, but in the SMP case we have less protection. pre-
* calculating the next head indexed does not eliminate all
* possibility of conflict; it is possible that one note could be lost
* or corrupted. But the integrity of the overal buffer should be
* retained.
*/
nxthd = note_next(head, notelen);
/* Write the next head. There are two possible race conditions: (1)
* some other CPU wrote the head after we fetched it above. That
* note from the other CPU will be lost (and this one may be
* corrupted), or (2) some other CPU will write the head after we
* write it and the write the same value. Again, one note will be
* lost or corrupted.
*/
g_note_info.ni_head = nxthd;
}
while (nxthd != g_note_info.ni_head);
/* Loop until all bytes have been transferred to the circular buffer */
@ -322,7 +350,7 @@ static void note_add(FAR const uint8_t *note, uint8_t notelen)
notelen--;
}
g_note_info.ni_head = head;
DEBUGASSERT(head == nxthd);
}
/****************************************************************************