linux-kernel - Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

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Message-ID: <20190211093601.GT32511@hirez.programming.kicks-ass.net>
Date:   Mon, 11 Feb 2019 10:36:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

On Sun, Feb 10, 2019 at 09:00:50PM -0500, Waiman Long wrote:
> diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
> index bad2bca..067e265 100644
> --- a/kernel/locking/rwsem.h
> +++ b/kernel/locking/rwsem.h
> @@ -32,6 +32,26 @@
>  # define DEBUG_RWSEMS_WARN_ON(c)
>  #endif
>  
> +/*
> + * R/W semaphores originally for PPC using the stuff in lib/rwsem.c.
> + * Adapted largely from include/asm-i386/rwsem.h
> + * by Paul Mackerras <paulus@...ba.org>.
> + */
> +
> +/*
> + * the semaphore definition
> + */
> +#ifdef CONFIG_64BIT
> +# define RWSEM_ACTIVE_MASK		0xffffffffL
> +#else
> +# define RWSEM_ACTIVE_MASK		0x0000ffffL
> +#endif
> +
> +#define RWSEM_ACTIVE_BIAS		0x00000001L
> +#define RWSEM_WAITING_BIAS		(-RWSEM_ACTIVE_MASK-1)
> +#define RWSEM_ACTIVE_READ_BIAS		RWSEM_ACTIVE_BIAS
> +#define RWSEM_ACTIVE_WRITE_BIAS		(RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
> +
>  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
>  /*
>   * All writes to owner are protected by WRITE_ONCE() to make sure that
> @@ -132,3 +152,113 @@ static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
>  {
>  }
>  #endif
> +
> +#ifdef CONFIG_RWSEM_XCHGADD_ALGORITHM
> +/*
> + * lock for reading
> + */
> +static inline void __down_read(struct rw_semaphore *sem)
> +{
> +	if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0))
> +		rwsem_down_read_failed(sem);
> +}
> +
> +static inline int __down_read_killable(struct rw_semaphore *sem)
> +{
> +	if (unlikely(atomic_long_inc_return_acquire(&sem->count) <= 0)) {
> +		if (IS_ERR(rwsem_down_read_failed_killable(sem)))
> +			return -EINTR;
> +	}
> +
> +	return 0;
> +}
> +
> +static inline int __down_read_trylock(struct rw_semaphore *sem)
> +{
> +	long tmp;
> +
> +	while ((tmp = atomic_long_read(&sem->count)) >= 0) {
> +		if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
> +				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
> +			return 1;

That really wants to be:

		if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
						    tmp + RWSEM_ACTIVE_READ_BIAS))

> +		}
> +	}
> +	return 0;
> +}
> +
> +/*
> + * lock for writing
> + */
> +static inline void __down_write(struct rw_semaphore *sem)
> +{
> +	long tmp;
> +
> +	tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
> +					     &sem->count);
> +	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
> +		rwsem_down_write_failed(sem);
> +}
> +
> +static inline int __down_write_killable(struct rw_semaphore *sem)
> +{
> +	long tmp;
> +
> +	tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
> +					     &sem->count);
> +	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
> +		if (IS_ERR(rwsem_down_write_failed_killable(sem)))
> +			return -EINTR;
> +	return 0;
> +}
> +
> +static inline int __down_write_trylock(struct rw_semaphore *sem)
> +{
> +	long tmp;

	tmp = RWSEM_UNLOCKED_VALUE;

> +
> +	tmp = atomic_long_cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
> +		      RWSEM_ACTIVE_WRITE_BIAS);
> +	return tmp == RWSEM_UNLOCKED_VALUE;

	return atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
					       RWSEM_ACTIVE_WRITE_BIAS);

> +}
> +
> +/*
> + * unlock after reading
> + */
> +static inline void __up_read(struct rw_semaphore *sem)
> +{
> +	long tmp;
> +
> +	tmp = atomic_long_dec_return_release(&sem->count);
> +	if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
> +		rwsem_wake(sem);
> +}
> +
> +/*
> + * unlock after writing
> + */
> +static inline void __up_write(struct rw_semaphore *sem)
> +{
> +	if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
> +						    &sem->count) < 0))
> +		rwsem_wake(sem);
> +}
> +
> +/*
> + * downgrade write lock to read lock
> + */
> +static inline void __downgrade_write(struct rw_semaphore *sem)
> +{
> +	long tmp;
> +
> +	/*
> +	 * When downgrading from exclusive to shared ownership,
> +	 * anything inside the write-locked region cannot leak
> +	 * into the read side. In contrast, anything in the
> +	 * read-locked region is ok to be re-ordered into the
> +	 * write side. As such, rely on RELEASE semantics.
> +	 */
> +	tmp = atomic_long_add_return_release(-RWSEM_WAITING_BIAS, &sem->count);
> +	if (tmp < 0)
> +		rwsem_downgrade_wake(sem);
> +}
> +
> +#endif /* CONFIG_RWSEM_XCHGADD_ALGORITHM */