On Thu, Dec 4, 2014 at 11:20 AM, Ola Liljedahl <[email protected]> wrote:
> Signed-off-by: Ola Liljedahl <[email protected]> > Reviewed-by: Bill Fischofer <[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 > > > _______________________________________________ > lng-odp mailing list > [email protected] > http://lists.linaro.org/mailman/listinfo/lng-odp >
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