It is expected that desc_read() will always set at least the
@state_var field. However, if the descriptor is in an inconsistent
state, no fields are set.
Also, the second load of @state_var is not stored in @desc_out and
so might not match the state value that is returned.
Always set the last loaded @state_var into @desc_out, regardless of
the descriptor consistency.
Fixes: b6cf8b3f3312 ("printk: add lockless ringbuffer")
Signed-off-by: John Ogness <john.ogn...@linutronix.de>
---
kernel/printk/printk_ringbuffer.c | 26 +++++++++++++++++++-------
1 file changed, 19 insertions(+), 7 deletions(-)
diff --git a/kernel/printk/printk_ringbuffer.c
b/kernel/printk/printk_ringbuffer.c
index 0659b50872b5..88f7dd4cb0c1 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -368,9 +368,9 @@ static enum desc_state get_desc_state(unsigned long id,
}
/*
- * Get a copy of a specified descriptor and its queried state. A descriptor
- * that is not in the committed or reusable state must be considered garbage
- * by the reader.
+ * Get a copy of a specified descriptor and return its queried state. If the
+ * descriptor is in an inconsistent state (miss or reserved), the caller can
+ * only expect the descriptor's @state_var field to be valid.
*/
static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
unsigned long id, struct prb_desc *desc_out)
@@ -383,8 +383,14 @@ static enum desc_state desc_read(struct prb_desc_ring
*desc_ring,
/* Check the descriptor state. */
state_val = atomic_long_read(state_var); /* LMM(desc_read:A) */
d_state = get_desc_state(id, state_val);
- if (d_state != desc_committed && d_state != desc_reusable)
- return d_state;
+ if (d_state == desc_miss || d_state == desc_reserved) {
+ /*
+ * The descriptor is in an inconsistent state. Set at least
+ * @state_var so that the caller can see the details of
+ * the inconsistent state.
+ */
+ goto out;
+ }
/*
* Guarantee the state is loaded before copying the descriptor
@@ -449,9 +455,15 @@ static enum desc_state desc_read(struct prb_desc_ring
*desc_ring,
*/
smp_rmb(); /* LMM(desc_read:D) */
- /* Re-check the descriptor state. */
+ /*
+ * The data has been copied. Return the current descriptor state,
+ * which may have changed since the load above.
+ */
state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
- return get_desc_state(id, state_val);
+ d_state = get_desc_state(id, state_val);
+out:
+ atomic_long_set(&desc_out->state_var, state_val);
+ return d_state;
}
/*
--
2.19.0
