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ptr_ring: document usage around __ptr_ring_peek

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This explains why is the net usage of __ptr_ring_peek
actually ok without locks.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Michael S. Tsirkin 2018-01-10 16:03:05 +02:00 committed by David S. Miller
parent d542296a4d
commit 66940f35d5
1 changed files with 10 additions and 4 deletions
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14
include/linux/ptr_ring.h
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@ -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);