The decision whether some more space is needed is tricky in the printk
ring buffer code:
1. The given lpos values might overflow. A subtraction must be used
instead of a simple "lower than" check.
2. Another CPU might reuse the space in the mean time. It can be
detected when the subtraction is bigger than DATA_SIZE(data_ring).
3. There is exactly enough space when the result of the subtraction
is zero. But more space is needed when the result is exactly
DATA_SIZE(data_ring).
Add a helper function to make sure that the check is done correctly
in all situations. Also it helps to make the code consistent and
better documented.
Suggested-by: John Ogness <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Petr Mladek <[email protected]>
---
kernel/printk/printk_ringbuffer.c | 31 +++++++++++++++++++++++++++----
1 file changed, 27 insertions(+), 4 deletions(-)
diff --git a/kernel/printk/printk_ringbuffer.c
b/kernel/printk/printk_ringbuffer.c
index 3e6fd8d6fa9f..ede3039dd041 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -411,6 +411,23 @@ static bool data_check_size(struct prb_data_ring
*data_ring, unsigned int size)
return to_blk_size(size) <= DATA_SIZE(data_ring) / 2;
}
+/*
+ * Compare the current and requested logical position and decide
+ * whether more space needed.
+ *
+ * Return false when @lpos_current is already at or beyond @lpos_target.
+ *
+ * Also return false when the difference between the positions is bigger
+ * than the size of the data buffer. It might happen only when the caller
+ * raced with another CPU(s) which already made and used the space.
+ */
+static bool need_more_space(struct prb_data_ring *data_ring,
+ unsigned long lpos_current,
+ unsigned long lpos_target)
+{
+ return lpos_target - lpos_current - 1 < DATA_SIZE(data_ring);
+}
+
/* Query the state of a descriptor. */
static enum desc_state get_desc_state(unsigned long id,
unsigned long state_val)
@@ -577,7 +594,7 @@ static bool data_make_reusable(struct printk_ringbuffer *rb,
unsigned long id;
/* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
- while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
+ while (need_more_space(data_ring, lpos_begin, lpos_end)) {
blk = to_block(data_ring, lpos_begin);
/*
@@ -668,7 +685,7 @@ static bool data_push_tail(struct printk_ringbuffer *rb,
unsigned long lpos)
* sees the new tail lpos, any descriptor states that transitioned to
* the reusable state must already be visible.
*/
- while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
+ while (need_more_space(data_ring, tail_lpos, lpos)) {
/*
* Make all descriptors reusable that are associated with
* data blocks before @lpos.
@@ -1148,8 +1165,14 @@ static char *data_realloc(struct printk_ringbuffer *rb,
unsigned int size,
next_lpos = get_next_lpos(data_ring, blk_lpos->begin, size);
- /* If the data block does not increase, there is nothing to do. */
- if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
+ /*
+ * Use the current data block when the size does not increase.
+ *
+ * Note that need_more_space() could never return false here because
+ * the difference between the positions was bigger than the data
+ * buffer size. The data block is reopened and can't get reused.
+ */
+ if (!need_more_space(data_ring, head_lpos, next_lpos)) {
if (wrapped)
blk = to_block(data_ring, 0);
else
--
2.51.1
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