Re: [dpdk-dev] [PATCH v1] eal: add ticket based reader writer lock

[Ruifeng Wang <Ruifeng.Wang@arm.com>]

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Stephen Hemminger
> Sent: Friday, January 15, 2021 1:35 AM
> To: dev@dpdk.org
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Subject: [dpdk-dev] [PATCH v1] eal: add ticket based reader writer lock
> 
> This patch implements a reader/writer ticket lock.
> This lock type acts like rte_rwlock() but uses a ticket algorithm and are fair for
> multiple writers and readers.
> Writers have  priority over readers.

The lock is ticket based to be fair. So writers should have no priority?

> 
> The tests are just a clone of existing rte_rwlock with test and function names
> changed. So the new ticket rwlocks should be drop in replacement for most
> users.
> 
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> ---
> Ps: I have additional tests for rwlock that test for fairness.
> Would these be valuable?
> 
>  app/test/autotest_data.py                     |   6 +
>  app/test/meson.build                          |   5 +
>  app/test/test_ticket_rwlock.c                 | 554 ++++++++++++++++++
>  doc/api/doxy-api-index.md                     |   1 +
>  lib/librte_eal/arm/include/meson.build        |   1 +
>  .../arm/include/rte_ticket_rwlock.h           |  22 +
>  .../include/generic/rte_ticket_rwlock.h       | 218 +++++++
>  lib/librte_eal/include/meson.build            |   1 +
>  lib/librte_eal/ppc/include/meson.build        |   1 +
>  .../ppc/include/rte_ticket_rwlock.h           |  18 +
>  lib/librte_eal/x86/include/meson.build        |   1 +
>  .../x86/include/rte_ticket_rwlock.h           |  18 +
>  12 files changed, 846 insertions(+)
>  create mode 100644 app/test/test_ticket_rwlock.c  create mode 100644
> lib/librte_eal/arm/include/rte_ticket_rwlock.h
>  create mode 100644 lib/librte_eal/include/generic/rte_ticket_rwlock.h
>  create mode 100644 lib/librte_eal/ppc/include/rte_ticket_rwlock.h
>  create mode 100644 lib/librte_eal/x86/include/rte_ticket_rwlock.h
> 

<snip>

> diff --git a/lib/librte_eal/include/generic/rte_ticket_rwlock.h
> b/lib/librte_eal/include/generic/rte_ticket_rwlock.h
> new file mode 100644
> index 000000000000..b3637358c1f7
> --- /dev/null
> +++ b/lib/librte_eal/include/generic/rte_ticket_rwlock.h
> @@ -0,0 +1,218 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2021 Microsoft Corporation  */
> +
> +#ifndef _RTE_TICKET_RWLOCK_H_
> +#define _RTE_TICKET_RWLOCK_H_
> +
> +/**
> + * @file
> + *
> + * Ticket based reader/writer lock
> + *
> + * This file defines an API for ticket style read-write locks.
> + * This types of lock act like rte_rwlock but provide fairness
> + * and requests are handled first come, first serviced.
> + *
> + * All locks must be initialized before use, and only initialized once.
> + *
> + * References:
> + *  "Spinlocks and Read-Write Locks"
> + *     http://locklessinc.com/articles/locks/
> + *  "Scalable Read-Writer Synchronization for Shared-Memory
> Multiprocessors"
> + *
> https://www.cs.rochester.edu/research/synchronization/pseudocode/rw.ht
> ml
> + */
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +typedef union {
> +	uint64_t tickets;
> +	struct {
> +		union {
> +			struct {
> +				uint16_t write; /* current writer */
> +				uint16_t read;	/* current reader */
> +			};
> +			uint32_t readwrite;	/* atomic for both read and
> write */
> +		};
> +		uint16_t next;	/* next ticket */
> +	};
> +} rte_rwticketlock_t;
> +
> +/**
> + * A static rwticket initializer.
> + */
> +#define RTE_RWTICKETLOCK_INITIALIZER { 0 }
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Initialize the rwticketlock to an unlocked state.
> + *
> + * @param rwl
> + *   A pointer to the rwticketlock structure.
> + */
> +__rte_experimental
> +static inline void
> +rte_rwticketlock_init(rte_rwticketlock_t *rwl) {
> +	rwl->tickets = 0;
> +}
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + * Take a write lock. Loop until the lock is held.
> + *
> + * @param rwl
> + *   A pointer to a rwticketlock structure.
> + */
> +__rte_experimental
> +static inline void
> +rte_rwticket_write_lock(rte_rwticketlock_t *rwl) {
> +	uint16_t me;
> +
> +	me = __atomic_fetch_add(&rwl->next, 1, __ATOMIC_RELAXED);
> +	rte_wait_until_equal_16(&rwl->write, me, __ATOMIC_ACQUIRE); }
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Try to take a write lock.
> + *
> + * @param rwl
> + *   A pointer to a rwticketlock structure.
> + * @return
> + *   - zero if the lock is successfully taken
> + *   - -EBUSY if lock could not be acquired for writing because
> + *     it was already locked for reading or writing
> + */
> +__rte_experimental
> +static inline int
> +rte_rwticket_write_trylock(rte_rwticketlock_t *rwl) {
> +	rte_rwticketlock_t old, new;
> +
> +	old.tickets = __atomic_load_n(&rwl->tickets, __ATOMIC_RELAXED);
> +	if (old.write != old.next)
> +		return -EBUSY;
> +
> +	new.tickets = old.tickets;
> +	new.next = old.next + 1;
> +	if (__atomic_compare_exchange_n(&rwl->tickets, &old.tickets,
> new.tickets,
> +					0, __ATOMIC_ACQUIRE,
> __ATOMIC_RELAXED))
> +		return 0;
> +	else
> +		return -EBUSY;
> +}
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Release a write lock.
> + *
> + * @param rwl
> + *   A pointer to a rwticketlock structure.
> + */
> +__rte_experimental
> +static inline void
> +rte_rwticket_write_unlock(rte_rwticketlock_t *rwl) {
> +	rte_rwticketlock_t t;
> +
> +	t.tickets = __atomic_load_n(&rwl->tickets, __ATOMIC_RELAXED);
> +	t.write++;
> +	t.read++;
> +	__atomic_store_n(&rwl->readwrite, t.readwrite,
> __ATOMIC_RELEASE); }
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + *
> + * Take a read lock. Loop until the lock is held.
> + *
> + * @param l

Nit, 'rwl'.

> + *   A pointer to a rwticketlock structure.
> + */
> +__rte_experimental
> +static inline void
> +rte_rwticket_read_lock(rte_rwticketlock_t *rwl) {
> +	uint16_t me;
> +
> +	me = __atomic_fetch_add(&rwl->next, 1, __ATOMIC_RELAXED);
> +	rte_wait_until_equal_16(&rwl->read, me, __ATOMIC_ACQUIRE);
> +	__atomic_fetch_add(&rwl->read, 1, __ATOMIC_RELAXED); }
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Try to take a read lock.
> + *
> + * @param rwl
> + *   A pointer to a rwticketlock structure.
> + *
> + * @return
> + *   - zero if the lock is successfully taken
> + *   - -EBUSY if lock could not be acquired for reading because a
> + *     writer holds the lock
> + */
> +__rte_experimental
> +static inline int
> +rte_rwticket_read_trylock(rte_rwticketlock_t *rwl) {
> +	rte_rwticketlock_t old, new;
> +	int success;
> +
> +	old.tickets = __atomic_load_n(&rwl->tickets, __ATOMIC_RELAXED);
> +
> +	do {
> +		uint16_t me = old.next; /* this is our ticket */

When __atomic_compare_exchange_n fails, old.tickets needs a reload.
 
> +
> +		/* does writer have the lock now? */
> +		if (old.read != me && old.write != me)

Check (old.read != me) should be enough?

> +			return -EBUSY;
> +
> +		/* expect to be the next reader */
> +		new.tickets = old.tickets;
> +		old.read = me;

This line is unnecessary?

> +		new.read = new.next = me + 1;
> +		success = __atomic_compare_exchange_n(&rwl->tickets,
> &old.tickets, new.tickets,
> +						      0, __ATOMIC_ACQUIRE,
> __ATOMIC_RELAXED);
> +	} while (!success);
> +
> +	return 0;
> +}
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Release a read lock.
> + *
> + * @param rwl
> + *   A pointer to the rwticketlock structure.
> + */
> +__rte_experimental
> +static inline void
> +rte_rwticket_read_unlock(rte_rwticketlock_t *rwl) {
> +	__atomic_add_fetch(&rwl->write, 1, __ATOMIC_RELEASE); }
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _RTE_TICKET_RWLOCK_H_ */
> diff --git a/lib/librte_eal/include/meson.build
> b/lib/librte_eal/include/meson.build
> index 0dea342e1deb..fe5c19748926 100644
> --- a/lib/librte_eal/include/meson.build
> +++ b/lib/librte_eal/include/meson.build
> @@ -65,6 +65,7 @@ generic_headers = files(
>  	'generic/rte_rwlock.h',
>  	'generic/rte_spinlock.h',
>  	'generic/rte_ticketlock.h',
> +	'generic/rte_ticket_rwlock.h',
>  	'generic/rte_vect.h',
>  )
>  install_headers(generic_headers, subdir: 'generic') diff --git
> a/lib/librte_eal/ppc/include/meson.build
> b/lib/librte_eal/ppc/include/meson.build
> index dae40ede546e..0bc560327749 100644
> --- a/lib/librte_eal/ppc/include/meson.build
> +++ b/lib/librte_eal/ppc/include/meson.build
> @@ -16,6 +16,7 @@ arch_headers = files(
>  	'rte_rwlock.h',
>  	'rte_spinlock.h',
>  	'rte_ticketlock.h',
> +	'rte_ticket_rwlock.h',
>  	'rte_vect.h',
>  )
>  install_headers(arch_headers, subdir: get_option('include_subdir_arch'))
> diff --git a/lib/librte_eal/ppc/include/rte_ticket_rwlock.h
> b/lib/librte_eal/ppc/include/rte_ticket_rwlock.h
> new file mode 100644
> index 000000000000..4768d5bfa8ef
> --- /dev/null
> +++ b/lib/librte_eal/ppc/include/rte_ticket_rwlock.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2021 Microsoft Corporation  */
> +
> +#ifndef _RTE_FAIR_RWLOCK_PPC_64_H_
> +#define _RTE_FAIR_RWLOCK_PPC_64_H_
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +#include "generic/rte_ticket_rwlock.h"
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _RTE_FAIR_RWLOCK_PPC_64_H_ */
> diff --git a/lib/librte_eal/x86/include/meson.build
> b/lib/librte_eal/x86/include/meson.build
> index 549cc21a42ed..e9169f0d1da5 100644
> --- a/lib/librte_eal/x86/include/meson.build
> +++ b/lib/librte_eal/x86/include/meson.build
> @@ -20,6 +20,7 @@ arch_headers = files(
>  	'rte_rwlock.h',
>  	'rte_spinlock.h',
>  	'rte_ticketlock.h',
> +	'rte_ticket_rwlock.h',
>  	'rte_vect.h',
>  )
>  install_headers(arch_headers, subdir: get_option('include_subdir_arch'))
> diff --git a/lib/librte_eal/x86/include/rte_ticket_rwlock.h
> b/lib/librte_eal/x86/include/rte_ticket_rwlock.h
> new file mode 100644
> index 000000000000..83c8bd0899d3
> --- /dev/null
> +++ b/lib/librte_eal/x86/include/rte_ticket_rwlock.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: BSD-3-Clause
> + * Copyright(c) 2021 Microsoft Corporation  */
> +
> +#ifndef _RTE_FAIR_RWLOCK_X86_64_H_
> +#define _RTE_FAIR_RWLOCK_X86_64_H_
> +
> +#ifdef __cplusplus
> +extern "C" {
> +#endif
> +
> +#include "generic/rte_ticket_rwlock.h"
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif /* _RTE_FAIR_RWLOCK_X86_64_H_ */
> --
> 2.29.2