printk: ringbuffer: fix setting state in desc_read()

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.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200914094803.27365-1-john.ogness@linutronix.de

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 0659b50872b5030f4b755ac2e6af8ca54b735d9f..88f7dd4cb0c1e0b24fe3cc21eaec67ef42b21259 100644 (file)
--- 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;
 }
 
 /*