Messages in this thread | | | Date | Mon, 9 May 2016 10:27:41 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field |
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On Fri, May 06, 2016 at 08:20:24PM -0400, Waiman Long wrote: > @@ -391,9 +386,11 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem) > * When there's no owner, we might have preempted between the > * owner acquiring the lock and setting the owner field. If > * we're an RT task that will live-lock because we won't let > + * the owner complete. We also quit if the lock is owned by > + * readers.
Maybe also note why we quit on readers.
> */ > + if (rwsem_is_reader_owned(owner) || > + (!owner && (need_resched() || rt_task(current)))) > break; > > /*
> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h > index 870ed9a..d7fea18 100644 > --- a/kernel/locking/rwsem.h > +++ b/kernel/locking/rwsem.h > @@ -1,3 +1,20 @@ > +/* > + * The owner field of the rw_semaphore structure will be set to > + * RWSEM_READ_OWNED when a reader grabs the lock. A writer will clear > + * the owner field when it unlocks. A reader, on the other hand, will > + * not touch the owner field when it unlocks. > + * > + * In essence, the owner field now has the following 3 states: > + * 1) 0 > + * - lock is free or the owner hasn't set the field yet > + * 2) RWSEM_READER_OWNED > + * - lock is currently or previously owned by readers (lock is free > + * or not set by owner yet) > + * 3) Other non-zero value > + * - a writer owns the lock > + */ > +#define RWSEM_READER_OWNED 1UL
#define RWSEM_READER_OWNED ((struct task_struct *)1UL)
> + > #ifdef CONFIG_RWSEM_SPIN_ON_OWNER > static inline void rwsem_set_owner(struct rw_semaphore *sem) > { > @@ -9,6 +26,26 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem) > sem->owner = NULL; > } > > +static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) > +{ > + /* > + * We check the owner value first to make sure that we will only > + * do a write to the rwsem cacheline when it is really necessary > + * to minimize cacheline contention. > + */ > + if (sem->owner != (struct task_struct *)RWSEM_READER_OWNED) > + sem->owner = (struct task_struct *)RWSEM_READER_OWNED;
How much if anything did this optimization matter?
> +} > + > +static inline bool rwsem_is_writer_owned(struct task_struct *owner) > +{ > + return (unsigned long)owner > RWSEM_READER_OWNED; > +}
Tad too clever that; what does GCC generate if you write the obvious:
return owner && owner != RWSEM_READER_OWNER;
> + > +static inline bool rwsem_is_reader_owned(struct task_struct *owner) > +{ > + return owner == (struct task_struct *)RWSEM_READER_OWNED; > +}
So I don't particularly like these names; they read like they take a rwsem as argument, but they don't.
Would something like: rwsem_owner_is_{reader,writer}() make more sense?
> #else > static inline void rwsem_set_owner(struct rw_semaphore *sem) > {
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