linux-kernel - Re: [PATCH v6 02/11] locking/rwsem: Implement a new locking scheme

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Date:   Wed, 11 Oct 2017 14:58:02 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-arch@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v6 02/11] locking/rwsem: Implement a new locking scheme

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