Signed-off-by: Ola Liljedahl <[email protected]>
---
When releasing a ticket lock, replace the atomic increment operation with
load-relaxed and store-release as this avaoids an (unnecessary) atomic RMW
operation which is expensive on some architectures.
Add descriptive comments for all ticketlock operations.
platform/linux-generic/odp_ticketlock.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/platform/linux-generic/odp_ticketlock.c
b/platform/linux-generic/odp_ticketlock.c
index 6c5e74e..682b01b 100644
--- a/platform/linux-generic/odp_ticketlock.c
+++ b/platform/linux-generic/odp_ticketlock.c
@@ -22,8 +22,13 @@ void odp_ticketlock_lock(odp_ticketlock_t *ticketlock)
{
uint32_t ticket;
+ /* Take a ticket using an atomic increment of 'next_ticket'.
+ * This can be a relaxed operation but it cannot have the
+ * acquire semantics since we haven't acquired the lock yet */
ticket = odp_atomic_fetch_inc_u32(&ticketlock->next_ticket);
+ /* Spin waiting for our turn. Use load-acquire so that we acquire
+ * all stores from the previous lock owner */
while (ticket != _odp_atomic_u32_load_mm(&ticketlock->cur_ticket,
_ODP_MEMMODEL_ACQ))
odp_spin();
@@ -32,7 +37,15 @@ void odp_ticketlock_lock(odp_ticketlock_t *ticketlock)
void odp_ticketlock_unlock(odp_ticketlock_t *ticketlock)
{
- _odp_atomic_u32_add_mm(&ticketlock->cur_ticket, 1, _ODP_MEMMODEL_RLS);
+ /* Release the lock by incrementing 'cur_ticket'. As we are the
+ * lock owner and thus the only thread that is allowed to write
+ * 'cur_ticket', we don't need to do this with an (expensive)
+ * atomic RMW operation. Instead load-relaxed the current value
+ * and a store-release of the incremented value */
+ uint32_t cur = _odp_atomic_u32_load_mm(&ticketlock->cur_ticket,
+ _ODP_MEMMODEL_RLX);
+ _odp_atomic_u32_store_mm(&ticketlock->cur_ticket, cur + 1,
+ _ODP_MEMMODEL_RLS);
#if defined __OCTEON__
odp_sync_stores(); /* SYNCW to flush write buffer */
@@ -42,6 +55,11 @@ void odp_ticketlock_unlock(odp_ticketlock_t *ticketlock)
int odp_ticketlock_is_locked(odp_ticketlock_t *ticketlock)
{
+ /* Compare 'cur_ticket' with 'next_ticket'. Ideally we should read
+ * both variables atomically but the information can become stale
+ * immediately anyway so the function can only be used reliably in
+ * a quiescent system where non-atomic loads should not pose a
+ * problem */
return odp_atomic_load_u32(&ticketlock->cur_ticket) !=
odp_atomic_load_u32(&ticketlock->next_ticket);
}
--
1.9.1
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