On 10/11/2017 02:40 PM, Peter Zijlstra wrote:
> 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).
Without fetch_add for readers, it could lead to reduced performance for
reader heavy workloads.
Are you trying to do a PI version of rwsem? It can work when the lock is
writer owned, but not when it is reader owned.
Cheers,
Longman
