On Wed, Oct 11, 2017 at 02:01:53PM -0400, Waiman Long wrote:
> +/*
> + * The definition of the atomic counter in the semaphore:
> + *
> + * Bit 0 - writer locked bit
> + * Bit 1 - waiters present bit
> + * Bits 2-7 - reserved
> + * Bits 8-31 - 24-bit reader count
> + *
> + * atomic_fetch_add() is used to obtain reader lock, whereas atomic_cmpxchg()
> + * will be used to obtain writer lock.
> + */
> +#define RWSEM_WRITER_LOCKED 0X00000001
> +#define RWSEM_FLAG_WAITERS 0X00000002
> +#define RWSEM_READER_BIAS 0x00000100
> +#define RWSEM_READER_SHIFT 8
> +#define RWSEM_READER_MASK (~((1U << RWSEM_READER_SHIFT) - 1))
> +#define RWSEM_LOCK_MASK (RWSEM_WRITER_LOCKED|RWSEM_READER_MASK)
> +#define RWSEM_READ_FAILED_MASK (RWSEM_WRITER_LOCKED|RWSEM_FLAG_WAITERS)
> +
> +#define RWSEM_COUNT_IS_LOCKED(c) ((c) & RWSEM_LOCK_MASK)
> +
> +/*
> + * lock for reading
> + */
> +static inline void __down_read(struct rw_semaphore *sem)
> +{
> + if (unlikely(atomic_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count)
> + & RWSEM_READ_FAILED_MASK))
> + rwsem_down_read_failed(sem);
> +}
So I implemented rwsem-mutex (also qrwlock based) that puts
(unsigned long)current | RWSEM_WRITER
in the atomic_long_t rw_semaphore::owner field. The down-side is that
you can't do fetch_add based __down_read, because that would clobber the
pointer. The up-side is that we have a stable owner pointer (which is
what I needed for PI like things).
I've yet to do performance tests -- and I've not done a bunch of the
obvious optimisations either.
