Messages in this thread | | | Date | Thu, 5 Jun 2014 09:22:48 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 1/1] remove redundant compare, cmpxchg already does it |
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On Wed, Jun 04, 2014 at 04:56:50PM -0400, Andev wrote: > On Wed, Jun 4, 2014 at 4:38 PM, Pranith Kumar <pranith@gatech.edu> wrote: > > remove a redundant comparision > > > > Signed-off-by: Pranith Kumar <bobby.prani@gmail.com> > > --- > > kernel/locking/rwsem-xadd.c | 3 +-- > > 1 file changed, 1 insertion(+), 2 deletions(-) > > > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > > index 1f99664b..6f8bd3c 100644 > > --- a/kernel/locking/rwsem-xadd.c > > +++ b/kernel/locking/rwsem-xadd.c > > @@ -249,8 +249,7 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) > > { > > if (!(count & RWSEM_ACTIVE_MASK)) { > > /* try acquiring the write lock */ > > - if (sem->count == RWSEM_WAITING_BIAS && > > - cmpxchg(&sem->count, RWSEM_WAITING_BIAS, > > + if (cmpxchg(&sem->count, RWSEM_WAITING_BIAS, > > RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { > > This was mainly done to avoid the cost of a cmpxchg in case where they > are not equal. Not sure if it really makes a difference though.
It does, a cache hot cmpxchg instruction is 24 cycles (as is pretty much any other LOCKed ins, as measured on my WSM-EP), not to mention that cmpxchg is a RMW so it needs to grab the cacheline in exclusive mode.
A read, which allows the cacheline to remain in shared, and non LOCKed ops are way faster. [unhandled content-type:application/pgp-signature] | |