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

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Date:   Wed, 11 Oct 2017 20:40:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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.