printk_ringbuffer: Create a helper function to decide whether more space is needed

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 <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/87tsz7iea2.fsf@jogness.linutronix.de
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20251107194720.1231457-3-pmladek@suse.com
[pmladek@suse.com: Updated wording as suggested by John]
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 3e6fd8d6fa9f2983bba70a5167af07d5e861d14e..56c8e3d031f4984e2edc6d47d542d09591bd9d9d 100644 (file)
--- 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 is 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 +1165,15 @@ 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, i.e.
+        * when @head_lpos is already able to accommodate the new @next_lpos.
+        *
+        * 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