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ptr_ring: clean up documentation 

The only function safe to call without locks
is __ptr_ring_empty. Move documentation about
lockless use there to make sure people do not
try to use __ptr_ring_peek outside locks.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
hifive-unleashed-5.1
Michael S. Tsirkin 2018-01-26 01:36:28 +02:00 committed by David S. Miller
parent 406de75554
commit 8619d384eb
1 changed files with 18 additions and 16 deletions
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34
include/linux/ptr_ring.h
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@ -169,21 +169,6 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr)
return ret;
}
/* Note: callers invoking this in a loop must use a compiler barrier,
* for example cpu_relax(). Callers must take consumer_lock
* 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)
{
if (likely(r->size))
@ -191,7 +176,24 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r)
return NULL;
}
/* See __ptr_ring_peek above for locking rules. */
/*
* Test ring empty status without taking any locks.
*
* NB: This is only safe to call if ring is never resized.
*
* However, 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
* re-test __ptr_ring_empty and/or consume the ring enteries
* after the synchronization point.
*
* Note: callers invoking this in a loop must use a compiler barrier,
* for example cpu_relax().
*/
static inline bool __ptr_ring_empty(struct ptr_ring *r)
{
return !__ptr_ring_peek(r);