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-rw-r--r--include/linux/ptr_ring.h14
1 files changed, 10 insertions, 4 deletions
diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index 6866df4f31b5..d72b2e7dd500 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -174,6 +174,15 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr)
* if they dereference the pointer - see e.g. PTR_RING_PEEK_CALL.
* If ring is never resized, and if the pointer is merely
* tested, there's no need to take the lock - see e.g. __ptr_ring_empty.
+ * However, if called outside the lock, and if some other CPU
+ * consumes ring entries at the same time, the value returned
+ * is not guaranteed to be correct.
+ * In this case - to avoid incorrectly detecting the ring
+ * as empty - the CPU consuming the ring entries is responsible
+ * for either consuming all ring entries until the ring is empty,
+ * or synchronizing with some other CPU and causing it to
+ * execute __ptr_ring_peek and/or consume the ring enteries
+ * after the synchronization point.
*/
static inline void *__ptr_ring_peek(struct ptr_ring *r)
{
@@ -182,10 +191,7 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r)
return NULL;
}
-/* Note: callers invoking this in a loop must use a compiler barrier,
- * for example cpu_relax(). Callers must take consumer_lock
- * if the ring is ever resized - see e.g. ptr_ring_empty.
- */
+/* See __ptr_ring_peek above for locking rules. */
static inline bool __ptr_ring_empty(struct ptr_ring *r)
{
return !__ptr_ring_peek(r);