On Thu, Aug 24, 2017 at 02:33:04PM +0200, Peter Zijlstra wrote: > On Thu, Aug 24, 2017 at 01:29:27PM +0200, Peter Zijlstra wrote: > > > > WTH did you not Cc the people that commented on your patch last time? > > > > On Wed, Aug 23, 2017 at 04:58:55PM +0530, Prateek Sood wrote: > > > If a spinner is present, there is a chance that the load of > > > rwsem_has_spinner() in rwsem_wake() can be reordered with > > > respect to decrement of rwsem count in __up_write() leading > > > to wakeup being missed. > > > > > spinning writer up_write caller > > > --------------- ----------------------- > > > [S] osq_unlock() [L] osq > > > spin_lock(wait_lock) > > > sem->count=0xFFFFFFFF00000001 > > > +0xFFFFFFFF00000000 > > > count=sem->count > > > MB > > > sem->count=0xFFFFFFFE00000001 > > > -0xFFFFFFFF00000001 > > > RMB > > > > This doesn't make sense, it appears to order a STORE against something > > else. > > > > > spin_trylock(wait_lock) > > > return > > > rwsem_try_write_lock(count) > > > spin_unlock(wait_lock) > > > schedule() > > Is this what you wanted to write?
And ideally there should be a comment near the atomic_long_add_return() in __rwsem_down_write_failed_common() to indicate we rely on the implied smp_mb() before it -- just in case someone goes and makes it atomic_long_add_return_relaxed(). And I suppose someone should look at the waiting branch of that thing too.. because I'm not sure what happens if waiting is true but count isn't big enough. I bloody hate the rwsem code, that BIAS stuff forever confuses me. I have a start at rewriting the thing to put the owner in the lock word just like we now do for mutex, but never seem to get around to finishing it. > --- > kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++ > 1 file changed, 27 insertions(+) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index 02f660666ab8..813b5d3654ce 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) > DEFINE_WAKE_Q(wake_q); > > /* > + * __rwsem_down_write_failed_common(sem) > + * rwsem_optimistic_spin(sem) > + * osq_unlock(sem->osq) > + * ... > + * atomic_long_add_return(&sem->count) > + * > + * - VS - > + * > + * __up_write() > + * if > (atomic_long_sub_return_release(&sem->count) < 0) > + * rwsem_wake(sem) > + * osq_is_locked(&sem->osq) > + * > + * And __up_write() must observe !osq_is_locked() when it observes the > + * atomic_long_add_return() in order to not miss a wakeup. > + * > + * This boils down to: > + * > + * [S.rel] X = 1 [RmW] r0 = (Y += 0) > + * MB RMB > + * [RmW] Y += 1 [L] r1 = X > + * > + * exists (r0=1 /\ r1=0) > + */ > + smp_rmb(); > + > + /* > * If a spinner is present, it is not necessary to do the wakeup. > * Try to do wakeup only if the trylock succeeds to minimize > * spinlock contention which may introduce too much delay in the
