[RFC] rwlock: prevent readers from starving writers

[Honnappa Nagarahalli]

<snip>
> 
> The original reader/writer lock in DPDK can cause a stream of readers to
> starve writers.
> 
> The new version uses an additional bit to indicate that a writer is waiting and
> which keeps readers from starving the writer.
This addition makes sense.
I am wondering if we should create a new lock. Is it possible that some applications are dependent on the current behavior?

> 
> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> ---
> Would like this to be in 22.11, but needs some more review
> 
>  lib/eal/include/generic/rte_rwlock.h | 93 ++++++++++++++++++----------
>  1 file changed, 61 insertions(+), 32 deletions(-)
> 
> diff --git a/lib/eal/include/generic/rte_rwlock.h
> b/lib/eal/include/generic/rte_rwlock.h
> index da9bc3e9c0e2..725cd19ffb27 100644
> --- a/lib/eal/include/generic/rte_rwlock.h
> +++ b/lib/eal/include/generic/rte_rwlock.h
> @@ -13,7 +13,7 @@
>   * This file defines an API for read-write locks. The lock is used to
>   * protect data that allows multiple readers in parallel, but only
>   * one writer. All readers are blocked until the writer is finished
> - * writing.
> + * writing. This version will not starve writers.
>   *
>   */
> 
> @@ -28,10 +28,17 @@ extern "C" {
>  /**
>   * The rte_rwlock_t type.
>   *
> - * cnt is -1 when write lock is held, and > 0 when read locks are held.
> + * Readers increment the counter by RW_READ (4)
> + * Writers set the RWLOCK_WRITE bit when lock is held
> + *     and set the RWLOCK_WAIT bit while waiting.
>   */
> +
> +#define RTE_RWLOCK_WAIT	 0x1	/* Writer is waiting */
> +#define RTE_RWLOCK_WRITE 0x2	/* Writer has the lock */
> +#define RTE_RWLOCK_READ	 0x4	/* Reader increment */
> +
>  typedef struct {
> -	volatile int32_t cnt; /**< -1 when W lock held, > 0 when R locks held.
> */
> +	volatile int32_t cnt;
>  } rte_rwlock_t;
> 
>  /**
> @@ -61,17 +68,24 @@ static inline void
>  rte_rwlock_read_lock(rte_rwlock_t *rwl)  {
>  	int32_t x;
> -	int success = 0;
> 
> -	while (success == 0) {
> +	while (1) {
>  		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
>  		/* write lock is held */
> -		if (x < 0) {
> +		if (x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)) {
>  			rte_pause();
>  			continue;
>  		}
> -		success = __atomic_compare_exchange_n(&rwl->cnt, &x, x
> + 1, 1,
> -					__ATOMIC_ACQUIRE,
> __ATOMIC_RELAXED);
> +
> +		/* Try to get read lock */
> +		x = __atomic_add_fetch(&rwl->cnt, RTE_RWLOCK_READ,
> +				       __ATOMIC_ACQUIRE);
> +		if (!(x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)))
> +			return;
> +
> +		/* Undo */
> +		__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
> +				   __ATOMIC_RELEASE);
>  	}
>  }
> 
> @@ -93,17 +107,23 @@ static inline int
>  rte_rwlock_read_trylock(rte_rwlock_t *rwl)  {
>  	int32_t x;
> -	int success = 0;
> 
> -	while (success == 0) {
> -		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
> -		/* write lock is held */
> -		if (x < 0)
> -			return -EBUSY;
> -		success = __atomic_compare_exchange_n(&rwl->cnt, &x, x
> + 1, 1,
> -					__ATOMIC_ACQUIRE,
> __ATOMIC_RELAXED);
> -	}
> +	x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
> +
> +	/* write lock is held */
> +	if (x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE))
> +		return -EBUSY;
> +
> +	/* Try to get read lock */
> +	x = __atomic_add_fetch(&rwl->cnt, RTE_RWLOCK_READ,
> +			       __ATOMIC_ACQUIRE);
> +
> +	if (x & (RTE_RWLOCK_WAIT | RTE_RWLOCK_WRITE)) {
> +		__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
> +				   __ATOMIC_RELEASE);
> 
> +		return -EBUSY;
> +	}
>  	return 0;
>  }
> 
> @@ -116,7 +136,7 @@ rte_rwlock_read_trylock(rte_rwlock_t *rwl)  static
> inline void  rte_rwlock_read_unlock(rte_rwlock_t *rwl)  {
> -	__atomic_fetch_sub(&rwl->cnt, 1, __ATOMIC_RELEASE);
> +	__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_READ,
> __ATOMIC_RELEASE);
>  }
> 
>  /**
> @@ -139,11 +159,12 @@ rte_rwlock_write_trylock(rte_rwlock_t *rwl)
>  	int32_t x;
> 
>  	x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
> -	if (x != 0 || __atomic_compare_exchange_n(&rwl->cnt, &x, -1, 1,
> -			      __ATOMIC_ACQUIRE, __ATOMIC_RELAXED) == 0)
> +	if (x < RTE_RWLOCK_WRITE &&
> +	    __atomic_compare_exchange_n(&rwl->cnt, &x, x +
> RTE_RWLOCK_WRITE,
> +					1, __ATOMIC_ACQUIRE,
> __ATOMIC_RELAXED))
> +		return 0;
> +	else
>  		return -EBUSY;
> -
> -	return 0;
>  }
> 
>  /**
> @@ -156,18 +177,26 @@ static inline void
> rte_rwlock_write_lock(rte_rwlock_t *rwl)  {
>  	int32_t x;
> -	int success = 0;
> 
> -	while (success == 0) {
> +	while (1) {
>  		x = __atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED);
> -		/* a lock is held */
> -		if (x != 0) {
> -			rte_pause();
> -			continue;
> +
> +		/* No readers or writers */
> +		if (x < RTE_RWLOCK_WRITE) {
> +			/* Turn off RTE_RWLOCK_WAIT, turn on
> RTE_RWLOCK_WRITE */
> +			if (__atomic_compare_exchange_n(&rwl->cnt, &x,
> RTE_RWLOCK_WRITE, 1,
> +
> 	__ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
> +				return;
>  		}
> -		success = __atomic_compare_exchange_n(&rwl->cnt, &x, -
> 1, 1,
> -					__ATOMIC_ACQUIRE,
> __ATOMIC_RELAXED);
> -	}
> +
> +		/* Turn on writer wait bit */
> +		if (!(x & RTE_RWLOCK_WAIT))
> +			__atomic_fetch_or(&rwl->cnt, RTE_RWLOCK_WAIT,
> __ATOMIC_RELAXED);
> +
> +		/* Wait until can try to take the lock */
> +		while (__atomic_load_n(&rwl->cnt, __ATOMIC_RELAXED) >
> RTE_RWLOCK_WAIT)
> +			rte_pause();
> +    }
>  }
> 
>  /**
> @@ -179,7 +208,7 @@ rte_rwlock_write_lock(rte_rwlock_t *rwl)  static
> inline void  rte_rwlock_write_unlock(rte_rwlock_t *rwl)  {
> -	__atomic_store_n(&rwl->cnt, 0, __ATOMIC_RELEASE);
> +	__atomic_fetch_sub(&rwl->cnt, RTE_RWLOCK_WRITE,
> __ATOMIC_RELEASE);
>  }
> 
>  /**
> --
> 2.35.1