On Thu, Jul 27, 2017 at 01:47:52AM +0530, Prateek Sood wrote: > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index 34e727f..21c111a 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -585,6 +585,40 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) > unsigned long flags; > DEFINE_WAKE_Q(wake_q); > > + /* > + * 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 > + * spin_trylock(wait_lock) > + * return > + * rwsem_try_write_lock(count) > + * spin_unlock(wait_lock) > + * schedule() > + * > + * Reordering of atomic_long_sub_return_release() in __up_write() > + * and rwsem_has_spinner() in rwsem_wake() can cause missing of > + * wakeup in up_write() context. In spinning writer, sem->count > + * and local variable count is 0XFFFFFFFE00000001. It would result > + * in rwsem_try_write_lock() failing to acquire rwsem and spinning > + * writer going to sleep in rwsem_down_write_failed(). > + * > + * The smp_rmb() here is to make sure that the spinner state is > + * consulted after sem->count is updated in up_write context.
I feel that comment can use help.. for example the RMB you add below is not present at all. > + */ > + 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 Your patch is whitespace damaged, all the indentation on the + lines is with spaces. Please resend with \t.
